When it's time for everyone to take a step farther in their life, it is usually when they decide to move out from their home where they've been living most of their life with their family. When this time arrives, one of the most common things to do is to share a flat.
Sharing a flat can be one of the best experiences ever, just because you are away from your family, and you start living with your friends who you've known for a different amount of time, but usually you've never lived with them, and that can crate uncomfortable situations or even some fights. On the other hand, living with your friends means that you have a connection with them in another level, they become your family and they are there not just for partying or hanging out, but for your day to day problems that you can’t share with your family because they don’t live there.
To conclude I think that the most important thing is to have some tight rules which everyone has to respect: who has to clean, when you can have your friends coming over or even who takes out the trash.