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.