One does not need a reason or excuse to reject an invitation he or she is not interested in.

I mean everything else outside of formality, by the way. Please give a reason when your boss asks why you can't handle a task. Other than that, and that's why I hate those people around me who asks me to go out and I say no and they demand for a reason and I don't have one. No mood is also a terrible excuse for some apparently. So, what are you going to do?