well there's a multitude of reasons.
my sister and i were recently in a shop where we were waiting to be waited on, and there was a man in front of us. this was like one of those cell phone places that sucks your very soul out. so anyway, we're watching the guy that was ahead of us...he was talking with the clerk guy for about 20 or 25 minutes. i kept glancing at his package surreptitiously and smiling because the whole time his fly was wide open. and for the record, it wasn't something i was internally making fun of him for, it was more like an amusing thing that helped the time pass by quicker. he caught me looking at one point and i smiled and looked away politely. he probably thought i was on the make or something, because he gave me a smirk. you would have thought that he'd have realized that his zip was open after my look, but no he didn't. i have no idea if he eventually caught it. i guess he did, he had to take off those pants at some point.
i don't really crotch-watch a whole lot, maybe a glance here or there but i'd never want to be rude so i don't openly look. if i do look and then i smile there could be any number of reasons why including but not limited to the fly being open, funny-looking pants, cool-looking pants, an obvious bulge coinciding with a randy mood, et al. maybe there's not even a reason, i have just happened to look at someone's crotch while simultaneously thinking of something else that made me smile completely not related to the crotch-watching.