Fear of Rejection
One reason it is so hard to talk about is that rejection is so often bound up with shame. It can lead you to feel like you are not attractive, or desired by your partner.
Fear of conflict
Again, as with the first example, when things don't get talked about, assumptions fill in the blank spaces. You end up assuming how the other is feeling and how they will react instead of giving them the chance to have their own reactions. Nothing sucks the life out of relationships like assumptions.
Fear of separateness
The common thread with these three examples is avoidance. It is an understandable way to protect oneself from experiencing what one fears. Avoidance implies not communicating. Not communicating means you are forced to make assumptions. This is why it is so dangerous to allow your fears to take control.
Facing your fears
Fear is like a bully or the bogeyman. The more you avoid it, the more powerful it becomes. There's no time like the present to stand up and face it!