Depression

I have many students who are plagued with melancholia.  I think it has always affected many people, but has recently been marginalized into a small subset of 'Bipolar' disorder.  I am fairly certain that the new and possibly even heavier volume of the DSM is changing the title -- again.
Many people (regardless of alphabet soup after the surname),  have postulated the possible reasons behind the disease.  Some range from genetics to parenting -- and now -- chemical changes within the brain.  It amazes me that not so many years ago, people would blame depression and mania and a score of other 'maladies' on the Supernatural.  It is possible.  Anything really is. 
It could also be that a depression or melancholy is natural.  Humans cannot have happiness all the time.  There are several chemical reactions happening in the body at all times and we cannot control them all.
Accepting that chain of events has helped me to deal with the tides of emotions.  Saying that depression is a state to run away from is like saying happiness is a state to run toward.  It creates attachment and that manifests a cycle of grasping.
Remember:  the lotus grows beneath the water, away from the light.