Basic idea:

If you are going to talk bad about him to me, why wouldn't you talk bad about me to someone else?

You might have personal bad feelings about your current employer or a specific member of your team because of the situation you were in. But from a professional point of view it is good to brush it away and focus only on the value you are bringing in.

Here are a few other reasons:

1. Nobody likes complainers.
2. Your new employer may know people at your current work place. You do not want to risk it, after all it's a small world.
3. Forgive and forget: Even if they had really trouble you.

Basic idea:

If you are going to talk bad about him to me, why wouldn't you talk bad about me to someone else?

You might have personal bad feelings about your current employer or a specific member of your team because of the situation you were in. But from a professional point of view it is good to brush it away and focus only on the value you are bringing in.

Here are a few other reasons:

1. Nobody likes complainers.
2. Your new employer may know people at your current work place. You do not want to risk it, after all it's a small world.
3. Forgive and forget: Even if they had really troubled you.