A question I'm sure a lot of us who now admit to being depressed (or have accepted the diagnosis if you want to look at it from the other perspective) have asked. I'm fairly sure my current depression comes largely from my life situation rather than anything fundamentally wrong with my brain. Hence you might describe it as just unhappiness, but as NaturalNatures says, when I get to the point I'd describe myself as depressed the unhappiness totally ruins my life and makes me irrational - I can't enjoy the things which I normally take great pleasure in.
I very definitely have ups and downs and when I'm on an up (which is thankfully the majority of the time) I find it hard to understand why I have such problems enjoying anything when I'm down, and when I'm down I find it hard to understand how I ever enjoy anything. Strangely though I can still recall and intellectualise the way I think when I'm in the opposite state, it's just understanding the emotions I have a problem with. I should probably also point out that whilst I am mostly not depressed (or on an up) I still have an underlying unhappiness which is maybe how a totally rational person (if there is such a thing?) would feel the whole time in my situation. Though the professionals I've spoken to always seem to diagnose me as depressed when I describe my feelings even when I'm smiling and generally happy - which has quite often been the case when I've seen them.
I suppose what I'm saying here is that depression is a bit more than just unhappiness. It might be triggered by an unhappiness with your life (I'm not sure I have experience of that not being the case), but goes beyond a rational reaction to that. Which does lead me to question how useful the drugs are to me when it's not cause by a fundamental problem with my brain (endorphins from exercise always seem to help me more - the only problem being that when depressed I have no motivation to go and generate any).
If there is a genetic aspect to this, I am probably more at risk of depression. My aunt drank herself to death (which was I presume related to mental health issues) and my father committed suicide. I guess I have previously had short lived bouts of mild depression when I didn't have such an obvious reason for it, though it was always very predictable and mostly related to having completed a goal and not set a new one, and until recently doing physical exercise was always sufficient to bring me out of it. Before anybody gets too worried about me, whilst I've thought about suicide from an intellectual perspective as one possible solution (hasn't everybody who's suffered from deep depression - or is it just me?) the idea has never appealed very much - there's always an alternative more attractive solution (not necessarily one I want to take, but the fact it's there is quite sufficient to save me).
Hmm I think I've gone way beyond the original question into self-therapy there - apologies, but I can't be bothered to edit out the irrelevant bits now. Just in case anybody is interested, at the moment I'm busy dragging myself out of a bout of depression, maybe even most of the way there as I don't think I've done much in the way of the sort of self pity I normally do when properly depressed.