Because 'banker' perfectly rhymed with 'wanker'.
John knew that he should not allow other people to get a rise out of him that easily. Hey, he lived with one of the most aggravating men in that city, didn't he?
Well, Sebastian Wilkes took the award of being more irritating than Sherlock Holmes on a bad day. If there was a registry of arrogant public school twits, Sebastian would be on it and have a five-star rating, to boot.
Had John ever thought Mycroft's way of speaking was slightly irritating? He couldn't remember, not really. Sebastian's pronunciation itself grated on John's already rather shaky nerves. The fact that his bloody credit card was rejected at the store in the morning and now he had to stand here and smile kindly when that... that suited twat...
Called Sherlock Holmes "buddy". Oh, no.
"This is my friend, John Watson," Sherlock's voice was tense and John's focus was back on his flatmate.
Wanker banker. Was that fucker making fun of Sherlock? Well, here's a bit of fun for you, mister. John allowed himself a thin, controlled smile.
"Friend, yes. Well, some would call us colleagues, I suppose," he said, slowly and as haughtily as he could. "But I think we would call it... partners. Right, Sherlock?"
Pale, bright eyes alighted on him, wide and startled.
"Partners, yes. This is... quite correct."
"Rii-ght," Sebastian looked at them suspiciously as they stood there, watching each other. John finally nodded towards their host, who was scratching his neck nervously.
Sherlock nodded mutely and they sat, Sebastian sending away the secretary and finally taking his place behind his desk. He watched them with a frown as they got comfortable on the soft chairs in front of him.
"So," Sherlock's voice still sounded a bit shaky. "So, you’re doing well. You’ve been abroad a lot."
John slowly extended his hand and put it on the backrest of Sherlock's chair. He traced Sebastian's eyes turning to him with interest.
"Well, some," the man replied absently.
"Flying all the way round the world twice in a month?"
Ah, here he is, the brilliant detective. My brilliant detective.
And Sebastian 'The Wanker' Wilkes laughed. He had the temerity to laugh at Sherlock's observing skills. And point at him.
"Right! You’re doing that thing," he turned to John, winking, as if trying to take him into confidence. "We were at uni together. This guy here had a trick he used to do."
Well, tell me something I hadn't guessed yet.
"It’s not a trick."
John's heart skipped a beat. There was something small and injured in that voice. That beautiful, sweet baritone should never ever be allowed to sound as if someone had kicked Sherlock. His lips pulled back in a silent snarl.
"He could look at you and tell you your whole life story."
Yeah, fucker, yours is probably rather simple. Just like your simple little mind. No wonder Sherlock lives in permanent state of being annoyed with the stupidity of general population. I wonder why he even bothered today. If it wasn't for the money...
John kept his mouth shut and his face straight, but his fingers made small, tight circles on Sherlock's back, warming up the little spot, feeling the taller man relax gradually under his ministrations.
"Put the wind up everybody. We hated him."
John leaned closer to the desk, trying to control his reactions, but from the corner of his eye, he saw Sherlock's face tightening as if in pain.
Oh no, not allowed.
"Well, your loss," he heard himself saying. "Let's get to the point, shall we? What do you need Sherlock for today?"
Wilkes' face twisted and he stood up nervously.
"Well, all business then, very well. We’ve had a break-in."