Mostly I don't show my cards. When I hit something, but thinking my opponent has the better hand, I don't show my cards, before I fold. The reason: When the other player bluffed, I don't want him to know he tricked me, cause otherwise he/she will do/try it again.
I also don't show my cards, when I just lost the game, because of the river, side card or things like that. Sometimes this happens more than once and my opponents think, I just call everything. When you later hit the best hand, usually the other players don't believe you and pay more money or even call an all.in.