I don't know if there's enough evidence to know if depression is caused by a difference in the brain, or vice versa.
But I know what you mean; it does sound trivial compared to people with "real" mental illnesses, partly because people say "I've been feeling depressed lately" flippantly, as opposed to someone with *with depression*.
For myself, I see it as self-perpetuating and quite simply flawed. If it's physical or just the result of habit, something is wrong with the way my thought process develops, so I call myself "depressed".
Also,
>>5538is right. People are even given pills if they just don't feel happy. Not being happy and being depressed are two completely different things (IMO).