What goes on in a guy's head?

What goes on in their heads when they're trying to get a girl to sleep with them?
I mean why do they waste their time on the girl who clearly won't give up their precious gift for them, why do they put in so much effort? It really baffles me, it does. I don't get why they just don't leave the girl and just find someone else who would be more willing to have sex with them.
I don't see why they make the effort to pressure a girl so much for sex when they quite clearly don't want to.
I was reading a book about this, on my way home...and the girl, despite the fact that she didn't want to have sex with the guy, she still loved him. It reminded me of myself last year, of how, despite the fact that he pressured me so much for it, I still really liked him.I won't say love because I was really deluded.
Jeez I was really stupid. I was considering it too, and I probably would have gone through with it, had he not gone all anxious-avoidant on me.
In a way, I'm glad he did though, I would have regretted it instantly, and would have had even more issues than I have today :S