My Reasons

Okay: Explanation time...

First and for mostly, it hadn't been Subaru's first idea, third actually, but it had all come about as a necessity for survival.

The why probably could start with events from two years prior:

Just after new year's, while still in his last year of middle school, something happened; dramatic and life altering, but Subaru had absolutely no recollection of it at all, he had apparently he'd been spotted near death, having apparently been in a terrible accident and fallen from a bridge in town.

Rushed to hospital and brought back from the brink of death, a few needed surgeries later, he then went on to stay comatose for a while, a few months actually, before eventually he pulled through and finally woke in a hospital bed.

He'd remembered the pain, the discomfort of being patched back together, undergoing several more procedures, which reconstructed his pelvis and upper legs, then came the hard and long slog of rehabilitation.

Long days and nights were spent in the hospital, learning how exactly to walk again was difficult, and against the odds, but Subaru didn't want to be a burden, condemned to a lonely chair and bed for the rest of his life, not that it was his fault or decision, but somehow Subaru had the perseverance and luck to push through his substantial injuries and start the long trek of resuming to a 'normal' life.

Life hadn't exactly been all that normal before either, being constantly adored as a 'little sister', by nearly everyone around him, as Subaru had quite the girlish stature and appearance, making his mother gush over the 'cuteness' and apply him in cutesy clothes - it had been a constant battle. Always the quiet, shy little flower, the spitting image of a 'dainty sister' whilst growing up, Subaru had been the 'little treasure' of the Sekiya family. Even his name hadn't helped - being constantly embarrassed by his family and nearly always wrongly assumed as a girl by many others had long dogged him during his early life.

Even in rehab at the hospital, some of the hospital staff had been wrong in assuming his feminine face and average, slender frame was that of a girl.

Sometimes even protesting, with his equally ambiguous sounding voice, had been to no avail, and so 'Subaru-chan' went on as reluctantly through most occasions, and apart from waltzing about starkers to prove he was a guy, which wasn't practical in the least, with some people he'd totally given up on establishing himself as a man to them.

After regaining the minimal use of his body, being able to walk for short distances, he'd finally been discharged and returned home, but it was just a change in scenery, nothing else happened quickly, for the next few months it was rare for Subaru to make the distance to the front door, let alone venture out of it.

Homeschooling had come about, mostly just supervised by either parent, who in retrospect had probably delayed their own careers at that point to care for their wounded child. Going back to school was well out of the question, and not really physically possible, so Subaru feel out of touch with most of his friends, which then started to affect his mental health too.

After a year, constant strength exercise and more determination lead to actual steps outside the house, but insecurities as well, too much time by himself and perhaps some depression had turned Subaru into a moody, uncaring phase, accompanying a bout of chunnibyo, saw off his last remaining guy friends, then with a little more time, gained strength and less supervision, Subaru wandered into the wrong crowd.

Sporting a samurai bob style of hairdo and dark unruly clothes, going by his self made nick-name of 'Saber' - Subaru simply slipped into a bunch of actual delinquents, who hung out all about the town, being able to join them to 'hang out', play games and generally waste time about on the streets, they became a part of his rebellious phase of life.

At first his down personality had matched-up with the gang-ish types, and they warmly included him in some of their little escapades and general contempt for the world. Fitting in with the little group was kind of nice in a way, a camaraderie, sense of belonging and similarities were nice, for a short time, then things escalated; more daring actions, outright theft and vandalism started to be prompted in order to stay with the gang, and suddenly this group wasn't all that 'nice' to be around anymore.

Circumstances soon changed at home too; both parents had accepted promotions to Tokyo, while Hitomi - his older sister, would join them, having graduated from high school and being successfully accepted to a university in the big city too. Subaru had decided to stay at home, even accepting the decision that he'd need to return to school, now that he had become able to. So shortly after the start of the year, everything was set and done, he had the house all to himself, which required more effort for his own - cooking, cleaning and tidying up after himself, but he could manage alright.

Already starting to distance himself from the gang, which Subaru had thought was just a small bunch of want-to-be delinquents, an incident occurred; some serious violence and some of the higher-up 'members' got 'nabbed'. While Subaru had been absent; back at home where none of his 'mates' knew his activities, apparently someone else snitched, spilling details to the police, who in turn arrested the actual perpetrators, but a mysterious someone pointed the blame at the absent 'Saber' and everyone else in the bunch jumped onboard.

First Subaru received a few threatening messages, then more upon more threats of violent unwarranted retribution, Subaru began to fear for his life, secluding himself inside again, and for a brief time it could work, and did, but after a while Subaru knew it couldn't last. Thankfully he'd never disclosed his home address to any of the gang, nor had anyone been invited or tagged along to his house either, so none had yet to find his dwellings either.

On one small venture to get food, Subaru spotted some of his former 'pals' suspiciously keeping an eye out, presumably for him, in the local mall. Evading them all the time would get very hard, if not impossible; they knew where to be and had relations with those who worked in the area, who in-turn knew others.

Fleeing to Tokyo would be impossible too; they usually hung about the station, so he'd be spotted for sure as he approached. Even getting deliveries that could sustain him at home for a while, was fairly risky, he knew at least a few of his former 'colleges' used to do deliveries from time to time for some cash, but with the new school term approaching, simply sheltering in the house alone just wouldn't be possible, it would be necessary to venture out into the world, which lead to one possible solution.

Disguise; it seemed pretty obvious as to how and as what, much to his own displeasure and uncomfortable feelings about getting into the cute clothes again, he would pass for a female better than anything else he could think of, so after a day or so to summon the courage to 'actually do it', Subaru - in some of his sisters modest fashionable items she'd left in her closet; a long dress, loose jumper and a fluffy scarf, he made a quick shopping dash.

Quietly, and as quickly as he could muster - which wasn't very, he made it to the nearest convenience store, just near the station, while it was fairly late in the afternoon; there had been a few people out in the streets. Nervous as hell, he kept his head down, very wary of being recognised, but with a touch of make-up, and letting his long dark-red hair flow, he did admit to himself that he made for a pretty girl, but it would be mortifying if anyone still recognised him as a guy.

If any of the gang caught him, it probably mattered less about what he was wearing, then who he was, after the onslaught of threats Subaru had received for his 'snitching', he'd probably end up dead either way - if they saw though his attempt of hiding his appearance.

As it so happened, there was no problem, he got to the store unscathed, bought a few items and retreated home without an incident, on his first run. A couple of days later, another quick shopping journey initially started the same, but while in the store, a few figures appeared to loiter about in the small car park right by, and surely enough after paying for the groceries Subaru recognised one of the men as he stepped out of the shop. Stunned with fear, horror and embarrassment all in one, standing alone in the doorway, unable to retreat or hide and with two hands filled with bags, his only option was to continue shuffling past the group.

Would they set upon him immediately...? Did they see straight through the guise...? How would they enact their revenge...? Would they follow him home and make the kill there...?

He'd barely realised in his terrified state that he'd arrived home, totally without fuss or pursuit, having his disguise fool them completely. After a few more nervous days without any unwelcome visitors, or watchers from the street, his nerves settled enough for him to calm down a little, and plan to attend his next appointment.

A meeting with some teacher had been required before Subaru could start on his second year of high school, which required making the first journey to the school, including a train ride to get there, quite the long journey for him now, but Subaru needed to go, otherwise questions would be asked, details brought about and so on.

Again passing through the station seemed successful, with no questions, stops or followers, he did get some glances on the train, that felt unnerving, but nothing became of them. Getting to the school took a little more time than anticipated, while he did carry a change of clothes in a small carry bag, Subaru didn't have the time to get into them, or the foresight to know where to do so either when he finally arrived, and upon entering the school's grounds he'd almost immediately been escorted to the faculty room.

They'll know instantly something's wrong, I'm dressed as a girl with make-up on to boot...!

Nervous and quiet in the presence of the teachers, no comment or unwanted mentions by the professional educators, while a couple of basic questions were asked, a few more little questions given to gage his abilities, and only one about the medical declaration of his inabilities to participate in anything physical, then a few follow up questions, such as 'will you be able to get to school okay?', then Subaru was clear to go, after being welcomed to attend.

Unbeknownst to Subaru; the vice-principal did make a mark to his official documentations - apparently crossing off the M for gender and switching it to W instead, to correct the 'error'.

Relieved, but a tad dumbfounded by passing the interview, Subaru made the return home without incident, now too tired and slightly more confident to completely ignoring some of his former gang mates near the local station. Even though one of the brash ones, who seemed completely fooled, tries a pick-up line, but was completely brushed off, as Subaru was totally oblivious - distracted with many thoughts about what had lay before him in order to keep up the ruse.

After just another day of contemplation; Subaru willed himself to go through with it, leaving himself with no real alternative, and so the school year began and he would dutifully attend, in his borrowed sisters uniforms, which he found two complete sets in a box packed up in storage.

With no doubt about it, everything fit rather well and after a good hard self-criticising look-over - while wearing the get-up, Subaru reluctantly admitted to himself that he really could be passable as a regular school girl - on the outside at least.