I agree that it is a very blanket statement for something that varies in over 7 billion humans around the word. Plus I have seen a chemical imbalance and that isn't a phenomenon that says "I want to feel depressed" that is a biochemical process that really is out of whack.

Very specific chemicals do very specific things in your brain (actually the process involved with addiction) and without this becoming a religious/political topic - it is all evolutionary for a reason. Even while dying you feel euphoria. It is a natural human reaction to soften your mind to what is really happening.

"Falling in love" is another one. Meeting someone new sets off a massive chain of chemicals that creates a euphoric feeling. This, of course, is simply to get us into bed and make babies Darwin's way lol

Everything going on in your body has a specific route and reason for it happening that way. In fact, depression (a natural chemical imbalance) could actually have a very clear evolutionary path. If it were to end up being a mutation that caused this imbalance in your brain chemistry......and this imbalance led to less human interaction....which led to less children in people with the mutation.....eventually that mutation would be taken out. And "positive" emotions would be the same (but opposite).

Sorry - I'm a science nerd. Now, of course brain chemistry isn't simply a mutation and has a lot to do with outside factors so even my example is a blanket statement. I guess that's why we have nerds like to who ENJOY studying this crap lol