Disclaimer: Nothing mine. Obviously.
He hadn't meant to take Sebastian's case. In the B. J. (Before John) era, he wouldn't have contacted Sebastian back. He wasn't after easy money. No, all he wanted to do was go there, parade John's existence – show him off to Sebastian who surely hadn't changed and was still a a stuck up prick – and then tell him that the case was too uninteresting for him to look into it.
All his careful plans had gone down the metaphorical drain. Sebastian questioned John's presence – of course he did, like he really had secrets anyone would have bothered to know – and Sherlock answered, "My friend." He wanted to see Sebastian's surprised expression – or his attempt to school his face, maybe.
Instead, John corrected him immediately saying, "Colleague," and it was Sherlock who needed to carefully blank his face, lest they find out how much the careless quip hurt him. So John and he weren't friends? He'd thought they were. John wasn't disgusted by him. He took care of Sherlock. He giggled with the detective. Wasn't that what friends were meant to do? Sherlock was confused. He bottled it all up. (And yelled, "Shut up!" to the Mycroft in his head who pointed out, "Who would want to be friends with a freak of nature, Sherlock?")
And somehow, in the midst of his confusion, the sleuth ended up promising Sebastian that he'd look into his case, which wasn't elementary as he'd expected (some embezzlement, maybe, he'd thought at the start). Who would be able to break in the bank despite the security system? And above all, who on earth would break in and limit himself to vandalizing one painting? What aim could that possibly serve?
It was a puzzle. As such, it was exactly the type of thing Sherlock enjoyed. Sebastian remembered him well from their university days. Not that it mattered if it was interesting. Sherlock still wouldn't let himself like working for his former acquaintance.
And more deeply mysterious that any riddle Sebastian could offer was no one else than John Watson himself. On the way home, he said casually, "So...someone broke in the impregnable bank. Are you sure it wasn't you? You could have easily flown and broken in by the window. And ruined things just for the heck of it. And because he was a twat and deserved it. 'We all hated him?' What the hell was that?"
His voice had started lightly teasing, but then it had suddenly become angry. Angry at Sebastian. On Sherlock's account. But not – emphatically not – Sherlock's friend. There was enough to make the detective's head spin with such contradictory behaviour.
Only he couldn't ask. He wasn't sure that he wanted John's answer. The "I'm getting angry because I'm a decent bloke, but of course we aren't friends. I wouldn't ever stoop so low."
So instead he replied conversationally, "It wasn't me. Sebastian isn't worth the hassle. And he was just being truthful, nothing more. Everyone hated me. I gave them no reason not to, mind."
"Bollocks. Hating you? They had no idea what they missed on," John countered, the sentence clearly heartfelt.
"Odd experiments in the lab, compulsive deducing and the joys of cocaine, mostly," the sleuth answered flatly. Uni hadn't been particularly happy – or at all. Sherlock didn't expect John's reply.
"Well, two out of three sound fun," the doctor said with a grin.
"You believe that you wouldn't have shunned me. Even back then," Sherlock stated, taken aback.
"We get along quite well, don't we? Though I'd have objected to the drugs," John replied. As if the idea of having his company ("Not friendship," Mind-palace Mycroft pointed out smugly) wouldn't have changed Sherlock's life at the time. Possibly. Probably.
"That we do," the detective agreed. What was true was true. "It's probably best that we didn't come across each other then, though. I'd persuade Mycroft to have you kicked out of the army, and you wouldn't have liked that."
"Why would you do something like that?" John replied, baffled and a bit hurt at such a prospect.
"Let you go where I couldn't follow with the chance that you got yourself killed on some godforsaken land? I wouldn't stand for it," Sherlock explained simply.
"But you could have followed, you know," the doctor pointed out with a smile.
"Me. In the army. Keeping my wings a secret from the multitude, and taking orders by some dimwitted sod. It'd go so smooth." Sherlock grimaced in distaste. But when John laughed, he joined in.