Economics of Happiness!

If you are going through a low, the base value for happiness is decreasing(analogous to the concept of diminishing utility in Economics). And, therefore, even a little good happening to you is going to make you feel a lot happier. 

On the contrary if you are on a high, the base value for happiness increases and, therefore, you need something big to happen to be happy.

Isn't this a good enough reason for you to be happy that you are going through a low :)