Disclaimer: I don't own Star Trek. It is the property of Paramount Pictures.
Bones was like the big brother Jim never had. Which was distinctly peculiar, as James T. Kirk did, in fact, have an older brother.
George Samuel Kirk was just four years Jim's senior, but for all the closeness they shared, there might as well have been decades separating them. When Jim was very young, he thought that was the way all siblings were: they lived in the same house, used the same bathroom, and had the same mom but otherwise had very little to do with each other. When Jim began school, he quickly discovered that this was not always the case. There were plenty of other boys his age who had brothers (or sometimes sisters) either younger or older than Sam, and they were not only relatives but also friends. They happily played together, ate together, and plotted mischief their parents would later scold them for together; hell, they even did their homework together, sometimes! Of course, everything wasn't all sunshine and daisies with Jim's friends and their siblings, but when they fought, inevitably, before the day was out, they were back to being buds again.
None of his schoolmates' brothers continuously looked at them like they were Klingon sludge that should be ejected into outer space.
That was when Jim got to thinking that it might just be his family that was like that. Maybe there was something in the Kirk genetic code that necessitated aloof distain? He didn't personally feel it, but maybe the gene in question just hadn't triggered yet. Everyone was always saying he'd understand stuff when he was older, so this was probably just one more thing that would hit along with puberty. Right? Yeah. So in a couple of years, he'd be able to just glance at his brother the way Sam glanced at him, and some deep part of him wouldn't ache anymore that the boy who should have been his closest confidant couldn't stand the sight of him. 'Cause then James wouldn't be able to stomach his brother's presence, either.
Then Frank and Johnny came and shot that theory to hell.
Frank was a complete and utter bastard, and for the life of him, Jim couldn't understand why his mother married the jerk. But he came with Johnny, and Jim guessed his new step-brother was okay. Sam apparently thought Johnny was more than just okay because he treated the older boy the same way Jim had always wanted Sam to treat him: like a true-brother, a soul-brother, not just a by-blood-brother.
That was when Jim realized the problem wasn't with his family; it was with him.
The day after this revelation, James Kirk drove his father's corvette off a cliff.
Mom was off planet at the time, Johnny was down the road at a friend's house, and Frank casually mentioned that he'd found a buyer for 'that damn antique' in the garage. Jim immediately lit into him, screaming that the 'vette belonged to George Kirk, Sr. and that he'd rather see it smashed to pieces than in Frank's slimy mitts. Before his step-father could gather the breath to yell back, Sam snarled, "Oh! Like you really give a fuck about Dad's things! You didn't even know him! You don't even remember him! It's your fault he's dead in the first place! If Mom hadn't been pregnant with you, Dad never would have gotten on some damn survey ship, and I'd still have a dad!!!" Obviously, James hadn't taken this well.
When Winona Kirk heard about the incident, Jim's subsequent booking at the sheriff's office, and Sam's attempt to run away, she decided everyone needed a breather. Jim got to spend his with his uncle on Tarsus IV.
Which was a can of worms the young man absolutely refused to reopen.
Jim came back to Earth, emaciated and frail, and Sam couldn't look at him at all, not even just to glance anymore. James wasn't sure if this was because his brother felt guilty or because he wished he'd died out there with 4,000 other colonists.
And Jim never found out either, as Sam successfully left home soon after, never to step foot in the state of Iowa again.
By then, Johnny had gone off to college, Mom had taken another away mission, and James was left alone with Frank. Jim wanted to be alone with his step-father about as much as he wanted a case of choriomeningitis, and Frank felt exactly the same. So, the teen wound up spending most of his nights out at bars and motels and wherever-the-fuck-else kept him away from home. And in one of those bars, James T. Kirk met Captain Christopher Pike.
The older man did not try to act like either a brother or a friend to Jim. He didn't call Jim 'pal' or 'buddy' or any other nickname intimating camaraderie that wasn't there, but he did call the him 'son'--just the once, as Jim wavered on the edge of unconsciousness, before Pike even knew who he was talking to. Frank had never once called Jim 'son.' And so, the young rebel actually listened to what Pike had to say when he came back to. The captain spoke the way Jim thought his real father would: with a mixture of disappointment, straightforwardness, and challenge. The young man didn't really like Pike at that moment (because really, nobody liked to be told they weren't living up to their dead dad's sacrifice) but Christopher did one more thing that George would have done if he'd been alive to do it: he gave Jim a true-brother, a soul-brother. He gave Jim Bones.
Not directly, of course. But if Pike hadn't seen Jim get the shit beat out of him in some backwater bar, and hadn't taken the time to be unduly parental and advice-giving and all, then Jim wouldn't have flopped into place on a Starfleet recruiting shuttle just in time for Leonard 'Bones' McCoy to announce, "I might throw up on you." And Kirk couldn't imagine life without the man.
After all, Bones was the big brother Jim never had.