Sharing a dorm room with a longtime friend may seem like the ideal situation, but you'd be surprised how difficult it can sometimes be to live in such close quarters with your BFF. Friendship comes with certain expectations that don't exist between two strangers. And just because you've been besties since the sixth grade doesn't mean you have identical needs when it comes to personal space and alone time.
Take sharing, for example. Suppose your friend subscribes to the "share and share alike" philosophy regarding clothes, food and, well, everything, but you like to keep some personal items personal. It's easier to set boundaries upfront with a roommate you don't already know than it is to create new boundaries with a roommate who's been your friend for years. Feelings get hurt, and before you know it, the two of you aren't speaking because she shared her apples with you but you have your granola bars locked in your desk.
Also, as we mentioned before, perhaps you want to start fresh and dispense with that old high school image. It can be harder to break free of your old self when an old friend bunks in the loft above your head.