I like salukigirl's answer.

Ok, I have sort of an answer to your main question. (If it is still open for debate... if not I'll share anyway).

I think happiness and sadness are like the temperature of life.

(Like hot and cold).

Touch something sad and you get cold... something happy and you get warmer... something like that.

And we're all trying to "find" stuff. Different stuff for all of us. And we're all bumping into each other. Playing hot and cold.

So anyhow I agree with your main idea. Depression is what tells us "turn around and go a different way." We don't always have to fight that feeing.

The only thing is... My legit depressive periods have always happened regardless of what is going on in my life.... what made it not just sadness was the extra "oomph" either trouble sleeping or eating or just brain stuff....

So...

But yeah, unhappiness is a cue something has to change!!!