He was basically a normal shy guy
The movie was based on an interesting premise and well executed.
But what I find interesting is that the guy (Doug) who spied on the girl was presented as quite a normal guy, not a freak or psychopath. I say he was normal because his main goal was to have a normal relationship with the girl. He wasn't in it for the peeping or terrorizing. He just wanted to get close to the girl. Sure his methods were somewhat irregular but, frankly, did he have a chance with that girl any other way?
An alternative would have been to turn the guy into a serial killer who stalks women to kill them. We have seen plenty of movies like that and I think it's good the present movie took a different direction.