Nothing turns me off a story more than poor dialogue. It has to sound real. When reading a story (be it a partial submitted here or a published novel) I can forgive the author for a lot of things. If the dialogue is unconvincing, it is very hard to win me back.
For example, the WRONG way to set up a situation:
“Josh and Mary are together again. I don’t think that’s going to work. He cheated on her before and people don’t really change.”
“I think you’re right. Mary is too dependent on him, too. She’s only going to end up in trouble again.”
A better way to say this could go something like:
“Mary’s back with Josh.”
“Seriously? God, I thought she was rid of him.”
“I know, me too. I swear, he smiles and she goes running back.”
People’s thoughts on a subject can be just as (or more) informative than straight information. In the second example, we already know the speakers don’t like Josh, and that Mary is infatuated. We don’t need all the information at once; it can come out later that they were separated because he cheated.