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There had been no conscious decision to be best friends; in the thoughtless way of children, it hadn't been any more complicated or deliberate than the immediate rapport that had sprung up between them on the train. It needed nothing more to carry the spark than the brightness of James' smile and the sharpness of Sirius' wit. When Sirius, in those early days in Gryffindor, had existed under the shadow of his name, James hadn't been motivated by any conscious drive: He had seen the thin line of Sirius' mouth, the resolute solitude in those grey eyes, and it hadn't taken anything more than that for James to attach himself to Sirius' side, proclaiming his general good qualities and his fitness for their shared House as if James were his own vaunted ancestor stumping for Muggle rights until the other Gryffindors forgot that Sirius was a Black and thought of him only as James'. It was only natural that they came to think of James as Sirius', too.
When Sirius, whose family was (in)famous for their interest in the Dark Arts, instead turned his talents towards Defense, that was also a purely unconscious decision. Perhaps, at the back of his mind, Sirius already knew that it was worth it: James had never mentioned it, hadn't said anything to Sirius directly, but his disdain for the Dark Arts and those who practised it was clear. It was a hard line for James, and while Sirius didn't understand it, he found discarding his family's practice surprisingly painless, as if it meant almost nothing at all. His parents didn't see it that way, of course, their son and Heir forsaking his blood-given talent, but what of it? If Sirius continued to practice the Dark Arts, it would have eventually cost him James' friendship, and no magic in the world could be worth that loss.
Even after their lives at Hogwarts expanded and their world had widened, that unspoken vow of support, of love, seemed to need no discussion, reinforcing itself through the actions of one in reaction to the other. James, raised to be honest and true, lied so easily for Sirius' sake that no professor could suspect him of falsehood; Sirius, who held no small amount of disdain for rules that yoked the extraordinary to the mundane, adhered to them, more-or-less, on James' behalf. Both boys had been shaped by their families to an unquestionable degree, and yet those family ties meant little when being bound by them might come at the other's expense: They made their own little family, choosing Remus and Peter and, above all else, each other. The bond that existed between Sirius and James had never been a conscious decision, not that moment on the Hogwarts Express, but everything that had come after? Those had all been choices, one after another, each made with the same instinctive, unshakable surety.
They would, in the end, always choose each other. It was as inevitable as they were inseparable.
… Right?