Depression

That depression is caused by a "chemical imbalance" in the brain seems a stretch.  The idea is one that's been highly profitable for drug makers, even if it rests on placebo effect. That depressed people have a different chemical balance in their brains when depressed doesn't seem that far of a stretch though. One of the easiest ways to change depression is through regular eating and exercise, often neglected because, well, if you're depressed, you probably don't feel like moving or preparing food.  Another very easy way to change it is through drinking--much easier.

Another weird issue is that depression can be caused by events that happened a long time ago.  That idea does seem to have some salience, but doesn't get to the basic valuation problem: when we see the same objective stimuli as other people, why do we evaluate it differently when we're depressed?

And a third idea, of course, is that depression is caused by immediate situations, and one's helplessness within those circumstances.

There's a unifying theme throughout, though,and it should be noted that all three causes can come into play. If you're depressed you don't see the future as very bright.  If I had to distill depression down into one thought, it would be that: you see the future as impossible, or impossibly difficult.