Why does all my dialogue sound stupid?

To continue from yesterday's questions posed by my friend, Heather, here is one about dialogue (my favorite thing to write!)

Heather: Why does all my dialogue sound stupid?

First of all, I laughed when I got this question, mostly because it probably doesn't sound stupid.

In my opinion, dialogue sounds "stupid" when:

a) It's not realistic enough.
b) It's too realistic.

Here are some things to think about...