I don't usually approach people in the library or when I'm getting food etc because I'm generally trying to get stuff done or just grab a bite to eat really quickly, but I've had people start chatting to me in the library. I don't like it. I get that they're just being friendly, but come on, I'm clearly trying to get things done. I have a really tight schedule though, so if I don't get things done when I've planned them, I have to lose out on sleep or not do them at all.
I like chatting to random people when I'm waiting for lectures and none of my friends are there yet or something. I just think there's a time and a place to make new friends, and a library desk isn't one of them.
edit: saying that, I'm never rude when people do chat to me in the library, I'm always pleasant. I just don't make much of an effort to get to know their life story.