take ecstasy?
haha.. sure to make u happy.. 
Just kidding.. I was also going to post that it may be your diet..
maybe your blood sugar going low, that make you feel low energy. I
also read from somewhere that there are a list of food that will
make you feel happy (can't rem which but i know Chocolate is one of
them)..
Or sometimes if you keep doing the same routine for a long time, u
may slowly start to get a burnout.
