When you want to smoke you have to do so with your smoking friends. This makes for a random grouping of people that get to talk about random things at random times. Or maybe you have a friend with whom you always relax. I started to smoke because I was bored. It was something to do between classes.
Cigarettes also help people deal with others.
Cigarettes have always been associated with being a nerve relaxer. Perhaps your professor has just piled on another project due before Spring Break, so you need to step outside and release some stress through inhaling and exhaling. Maybe you and your roommate are fighting a perfect time to step away from the situation, breathe in some smoke and think about the solution to your troubles. While it may not be as rebellious as it used to be in high school and you may not have to sneak around and hide it from people, it is still rebellious to smoke. I smoke now because I'm addicted and I enjoy it.