When people say "yes," they aren't necessarily agreeing with you. Culturally, we're conditioned to be friendly and for many people saying "no" can feel uncooperative or unkind. As a result, when people say "yes" it's hard to tell whether they are trying to placate you or whether they actually agree with you.
Never Split the Difference does a great job of explaining why you're better off pushing your counter-party to say "no" before you finalize an agreement. When people says "no" you know with a greater degree of certainty that they're sharing their true preference and what their motivation for saying "no" is. When people say "yes," it's much harder to tell.
With a "yes" before a "no," you may be leaving value on the table or running the risk that your counter-party will back out of your agreement later.