I think too many people go into a roommate situation blind. They assume that since they get along with each other, they can live with each other. Which is not true. You have to consider your individual personalities and whether they would clash.
My two college roommates were two of my best friends in high school. We set out the rules in advance and made the decision that any other conflicts that came out had to be settled among the three of us. One of the most important decisions we made was that no one could have an overnight guest without permission of the other two.

Some types should never live with each other. If you are a loner or a person who is uncomfortable sharing your stuff, who should probably not live with someone who is outgoing. I could have never lived with someone who had kids. The noise and distractions would have bothered me no end.