Because you can reach a perfectly amicable point where you realize that one person's belief is a decision - you understand their arguments, they understand yours, but nothing that either of you say will change the other person's mind. We tend to think that people think through the options and go for the most salient one, but sometimes they say, "I'm going to believe this" and then they do. In that case, you'd have to unseat their will to change their mind.