Why are we offended?

We are offended only when we have unreasonable expectations that weren’t destined to come true, and that are difficult to refuse. For example, someone has agreed to spend a pleasant intimate evening with you, and the day before (or even half an hour after the appointed time) they call and say that they feel sick and won’t come... A familiar story? It’d seem that the reason is valid, but how much resentment rises!