Because they were hypocritical about it. Let's say you're afraid that someone might punch you in the face. So that person gets on a soapbox and gives an eloque

Because they were hypocritical about it. Let's say you're afraid that someone might punch you in the face. So that person gets on a soapbox and gives an eloquent speech to a crowd about how they're not going to punch you in the face, in fact, they're quite against face-punching and regret the times in the past they've punched people, and you're able to relax a little. And then three days later they punch you in the face anyway. Don't you think that punch might hurt a bit more, on an emotional level, than if that person just clocked you when you thought they would?