I've been wearing black non-stop for the past 30 years. I started as a Goth, but this is not a reason in itself; why does one want to be a Goth anyway?
I moved away from Gothic paraphernalia because I do have a sense of ridicule and I am getting old, but I still wear black all the time. In Summer I may make concessions and wear one - just one - item in tan, grey or army green.
Why I wear black: definitely not for the slimming-effect myth, which is just a myth. I've been all sizes in life, and being thin or fat in black is exactly the same as being thin or fat in colours.
What I do know is that it feels like second nature. I do own a couple of non-black outfits for important occasions (like meeting a new client who may have conservative outlooks; I'm self-employed, image matters), but when I wear them I feel like you would feel if you were forced to walk around in a Teletubbies costume, NOT on Halloween: just wrong.
Oh, by the way, the "no need to match clothes" is also a myth; all-black wearers are terribly fastidious about the different hues of black and about what fabric matches with the others, so yes, sometimes we can spend the same time as you do figuring out an outfit that matches.
I am also not depressed, not sad, not negative, I'm actually a rather upbeat person and I love life, so I'm not wearing black to express any conscious pain. (It may be subconscious, but I always thought it wasn't worth bothering a shrink about it).
I know several people who are exactly like that. And yes, you rarely find people who spend a lifetime dressing only in white, or only in turquoise. There is something about black that feels protective and familiar, so much that you're happily willing to put up with all the Morticia jokes and the "Oh but you have such a pretty face, you should try wearing pink/florals/pastels, they'd brighten you up!".