As Chris Voss describes in Never Split The Difference, when going into a negotiation, have a few hypotheses in your mind about what the person your negotiating is thinking/wants. Don’t assume you know what they want. I’ll spare you the old saying about assumptions.
Armed with several hypotheses in mind, rather than assumptions, as you enter a negotiation, begin gathering information from your counterpart to “test” these hypotheses. As you learn more you can continue to test different hypotheses until you land on the correct one or ones.
When you assume, you’ve already made up your mind about what the other party wants, when you actually have no clue.
I don’t want to be an ass, and you probably don’t either.