While a lot of diagnosed people can deal with mental illness on their own, I think there's a range and the more extreme cases are largely physical, not environmental or social.

I've read a little bit about drugs like lithium and other antipsychotics, and they measure brain mass or how dense the dendrites are, and generally, if someone is having psychotic episodes, and they take these drugs, it seems to reduce the loss of neurons.  (By psychotic episode, I mean hallucinations or times when people lose control of themselves.)

That seems logical to me.  We're always losing neurons, and our brains adapt by reinforcing things we wish to remember.  As we age, our thinking slows down -- as a middle aged person, I can attest to this -- but we remain intelligent and functional largely because we simply ignore a lot more stuff that we think doesn't matter.

So, my point here - some mental health is a physical, degenerative disease.  Let's not get all mystical about it.  It's just the brain losing matter.  I suspect it's analogous older people losing muscle mass, or bone mass, or hair, or other cells.  That's why old people suffer dementia more often than young people.

I don't know why so many teens get morose and into moody, dark music, like I did, but my inclination is to think it's something related to brain development.  Maybe it's a combination of physical, social, and environmental factors.