The Inner Fire is to Light Up All That Darkness.

People who are creative are also prone to depression or mood disorders.

This is not to say that emo leads to great art. We have deviantArt as proving grounds against that. Instead, the Fire in the Head that is often referenced for artists is possible in response to them being able to over-study input from the environment, something that coincides with depression or bipolar disorder or whatever.
Considering how often it was referenced in stories and characterizations of artists, it's nice to see there's a study backing this idea up. Me, I always suspected the overly sensitive were a bit prone to recreating the world that was grating on them.