She had to be some kind of illegal substance.
Sure he had crushes in highschool. Despite what Beca might think, he isn't a total nerd. But none of them were like this. All those girl were pretty, but eventually got over them. Not this girl. He physically could not bring himself to stay away from her.
She's rough around the edges and quite honestly she can be downright mean. Shehadn't a temper that flares up almost as frequently as Bumper acting like an ass. She pushes people away like there's no tomorrow. She makes him feel horrible whenever he's done something to piss her off. He sometimes thinks that this girl might actually be bad for his health (and definitely his school work.)
But he can't help it.
Because to Jesse, Beca is the one thing he doesn't think he could go without. For every flash of anger she will show a hint of a softer side. Each time she pushes him away she will give him what he calls "the look," which he interprets to mean she wants him to stay. And every time he tries to annoy her (because she's just so damn cute when frustrated) she always has a come back ready to go.
She keeps him on his toes.
Basically Beca is his equivilant to crack.
And with this realization he couldn't help but laugh out loud,imagining what her reaction would be if he told her that she's his own personal stash of cocaine.