How often do you feel the need to explain yourself?

This afternoon someone asked a favor of me and because of some very good reasons I said no. Once he got my response he reacted with a kind of smart alec-ish “um ok.” As soon as I read that I wanted to fall all over myself explaining why I couldn’t do the favor and apologizing for not being able to. Even as I type this question I feel like I need to justify why it was ok for me to refuse this favor! How often to you find yourself feeling compelled to explain your actions/decisions?