"What do you say about those that say that happiness is all in the mind?"

Tough question. Really one for philosophy and religion as much as medicine. If happiness were a formula, it would sure simplify a lot. But no doubt, attitude has a lot to do with it.

Specifically about depression, that's a very complex issue. I'm not a doctor or a therapist, just an observer and a veteran of depressive episodes.

Some simplified comments:

I've always thought that there's nothing wrong with being depressed if you've got something to be depressed about. Job loss, death of a loved one, health problems (staying upbeat after my stroke wasn't easy), etc., can be depressing situations. People get knocked down and it can be hard to get back up, but most manage it.

Chronic depression -- getting into a rut and staying there -- *can* become a habit. In that case, it can be "in someone's head." (Not forgetting medical causes, of course.)

Just my opinion, but a lot of chronic depression has some pretty nasty causes, including selfishness, anger, laziness, self-pity, etc. Depression can be a luxury; one of the most depressed people I knew really had nothing to be so depressed about, frankly.

Two women I knew: One was pretty spoiled and spent most of her life feeling cheated by life and was miserable. Another woman was orphaned as a child; survived a Nazi slave labor camp as a teenager; later married and had 12 (!!) children; had an abusive alcoholic husband; had to move across the globe and start a new life in middle age; worked like a dog every day of her life; survived bad health. Yet she found joy in life and was actually a pretty happy person overall.

Why such different outcomes? I have no idea. Faith in God, maybe? Gratitude? Biology?

If I knew the answer to all this, I'd be rich. And I'm definitely not rich!