Sirius knows there is a fine line between affection and obsession. He wonders if he's crossed it.
Whichever it is, Sirius blames it partially on James, because it's not Sirius's fault James has a tendency to wander around shirtless. It's nearly impossible not to look at his body.
And it wasn't Sirius's fault that James was his first kiss. It had been James's suggestion, probably, and if not that, he agreed to it! That wasn't Sirius's fault.
Practice kissing… Sirius snorts indignantly. That was his most bollocks idea ever. It turned him into a psychotic!
James is head over heels in love with Lily, but Sirius is afraid his kind of obsession isn't as adorable, and surely not as accepted.
But at least he isn't alone in his obsession. Sirius knows Peter is mad for James too. Sirius likes to think he has a better chance then the rat. Hopefully.
And then there's Remus, who doesn't seem to fancy anyone. Sirius is jealous.
But he'll have to ask Remus his secret later because James just announced he's taking a shower and Sirius feels a bit dirty as well.