It's funny -- to take a slightly more serious slant to ceo's funny post -- I find the tendency when feeling "flat" is to go to harder and harder porn. But that can actually have an anesthetizing effect . You get numb to it and then nothing works.
Give it a break for a while and then suddenly a nice ass walking down the street gets you going. See a pair of tits and you're popping wood. It just seems to work that way.