There is a philosophical reason for the "sadness" factor being the most relevant creatively speaking, and I'll tell you why:
Happiness is static motion. Happiness is destination, happiness is the "end of the road." Happiness is being content, is being satisfied. Content and satisfied is static motion. Static motion is philosophical death. Life is a journey, not a destination.
When you're sad, when things suck; you seek to improve. You question, analyze, interpret, decide, move. You seek the destination, you fight, you struggle; you're mortal and you need to move on.
There is no reason to discuss, analyze, or seek betterment when you are content. Being content is the destination, it's what you want. This is no movement of the mind or of anything. You may as well be dead.
Sadness and melancholy are true mortal emotions. Find beauty in the dissonance, and know that life is meant to be doomed to failure. Find comfort in the fact that you will never be truely happy; that your purpose is to move with depression, not to stand still with contentness.