I tried to post this to the TCS list, but it was rejected. -sigh- Anyway, enjoy:
The better you know someone, and the better they know you, the more intimate things it is safe to tell them. Which meshes amazingly well with a gradual approach to relationships, and extremely poorly with any sort of discontinuous jump.
By the way, this is important to parents who've messed up in the past, and now have an older child but little relationship. "Come tell me all about you, so we can catch up," would be just the wrong thing to say.