1. Their habits becomes yours.
You start to become the person you share a room with without noticing your changing. Everything changes since you're around them so much. When they sleep is when you sleep, what they watch is what you watch and even what they say starts to become what you say.
2. Somethings are better left unsaid.
If you have nothing nice to say, don't bother saying it. Don't cause issues that won't matter the next day. It's not worth it. Just suck it in and suck it up.
3. On another note, some things need to be said.
If you have a problem that there is a solution to, talk it out instead of holding it in. When you hold problems in, they escalate and then you explode. If your roommate is being messy or snoring too loud, let them know. Who knows, maybe they'll thank you/
4. Respect
You start to respect the people you share a room with on a new level. Since you are always around the you start to respect their opinions even when they collide with your own.
5. What's mine is theirs.
When you share a living space, you end up sharing everything. You find yourself asking to borrow things but over time, you just stop asking and start taking. They do the same thing. Whats yours becomes theirs and whats theirs becomes yours. Everything is shared, except the toothbrush of course.