Depression isn't just /like/ physical pain. It can cause real, physical pain. Pain, for which you go to doctors who say "oh, you're fine." Pain for which you go to dentists only to hear "oh, you're fine."

In many (most? all?) people depression has a physical manifestation. It hurts. It aches. It's not a "hole in your heart," it's like having the flu. All the time. It's not like a toothache. It is a toothache. This distinction has to be made because it's important: the pain is real - even if it is just created by brain chemistry.

Suicide, ingestion of drugs (incl. alcohol), addiction, religious fervor - all of these can be thought of as ways to escape the weight of "the self". A more abstract theory than "neurotransmitters being out of balance", but more useful perhaps in understanding what compels a person to act in those ways. People who commit suicide out of the blue, surprising friends and family, probably felt they were living a lie in some way, and couldn't see any way to get out of that other than escaping it entirely. If someone says "but they seemed to have it all under control" then ... how does anyone know what goes on in the head of another person?

I'm surprised I see no mention of the poem "Richard Cory" whenever this topic arises

http://www.poemhunter.com/poem/richard-cory/