Originally Posted by Cannibal
My mood usually dictates that decision. Sometimes I just don't want to risk the chance of having company if I'm out there to ponder the universe and such. Other times a little company is nice. Sometimes I just find a place that 'feels' good, so I make camp. If I'm with a group then the campground is a good place to meet and usually laziness kicks in if you've been waiting for someone, so I'll hang there.
You forgot about the "Heck, this site will do, let's hang here!" (Used late in the afternoon when a surprise rain is starting up.)