You’ve no doubt heard that emotions have no place in a negotiation—that you should check your emotions at the door and remain calm, cool, and collected throughout. But there’s a problem with this advice: it’s really quite useless. In negotiations, emotions are inevitable for three simple reasons:

First, you’re a human being, not a legal machine, so you’re almost certainly feeling something.

Second, you have limited control. You can’t ever know for sure how your counterpart will respond to...