1. You know what they think
You don't have to make an effort to learn their thoughts about something.
2. Sex becomes enjoyable
In the end he/she was the one to whom you were telling everything...even your sex life.
3. You wish there were 25 hours in a day
You want to do things that you weren't able to do while you were friends.
4. You can mock at each other
And no one takes offence.
What else? Please comment.