"If you were happy all the time, you wouldn't be human. You'd be a game-show host."
--Veronica (Winona Ryder) in Heathers
The quote above very much represents a common way of approaching mood in gothdom. We're not depressed all the time. It just seems that way sometimes because our culture at large tends to discourage expression of any dark or negative emotions. There are goths who choose to play the role of the tragic, depressive artiste-martyr ("oh, the angst!", said in a tragic voice with the back of the hand placed artistically against the forehead; this pose is known in goth shorthand as "Hand.Staple.Forehead."), but by no means is every goth like this. Most of us, in fact, find this pose to be ripe for ridicule. A certain darkness of character and emotion is common in gothdom, however, though not constantly manifested. There are a few reasons for this:
1. A sort of "fashinable angst," a reaction to our culture which tells people that being constantly happy and never showing distress is the only acceptable way to behave. A form of rebellion.
2. A fascination with the forbidden, the darker side of life, which is generally thought of as distasteful and disturbing by the culture at large. Although this can once again be rebellion, there are also people who are genuinely interested in this, by nature, and enjoy exploring it both for the sensations it carries and to examine why it exists and why we keep trying to deny it.
3. A tendency to be extremely analytical combined with a high degree of emotional sensitivity and empathy, which often causes despair at how ugly and cruel life can be. This kind of personality is frequently drawn to gothdom, probably because this is also the kind of personality that frequently makes an artist, and artists are welcomed in goth culture. For younger goths, the despair is usually focused on themselves (but not always), while for older ones it's more frequently focused on the plight of others (but not always). These people frequently want to make things better but find themselves stumbling over their own over-analyzing and getting frustrated by the difficulty in making progress.
Goths are not more prone to true depression or any other form of mood disorder. We do tend to be emotional and expressive, and thus our dark moods will be more noticeable--but then so will our bright moods. Goths are much more likely to express their tendency to darkness in morbid humor. That, however, seems to be the only thing that's less appreciated than a dark mood. Oh well. We'll just have to suffer through the torment of it all...