A young friend of mine asked me if I was interested in going to a mutual friend's house one evening to play Munchkin. There were several reasons I could not, but one was "it's too silly". Munchkin is a deliberately silly game. This is amusing for a little while, but after that it just gets in the way.
Yet, when I was playtesting one of my zombie games I said to the players, "it's a silly zombie game after all". But the silliness is of a different kind, and I asked myself what made the difference.