I've wanted to try and write something about depression for a while now, having recently been affected by it myself and before that having had it affect people very close to me. I can normally find a way to write about almost anything, but it stumps me every time. No description I've ever come up with quite captures it fully. However, I will try. This may be a bit of a wall of text depending how "into" this I get, sorry in advance if it is.
The light switch analogy that qarky raised is a very good one, particularly because the turning point really can be as instant as the flick of a switch sometimes. Happiness to sadness, self-loathing, anger or, worse still I think, just... nothing, total apathy, in almost literally the blink of an eye. At first it really can come when you least expect it, though I say "at first" because from my experience there comes a point where you're just always expecting it, you know it's gonna happen eventually.
It's got a particular little nasty twist to it for me being diagnosed Aspergers - social situations are difficult enough at times as it is without that switch flicking off and making things harder. Hell, sometimes the difficulty of social situations is what flicks the switch, it definitely has happened, so there's a vicious little cycle there, even.
With all of that said, my personal experience of it has thankfully been seeing the signs of it in myself relatively early (with a little help from my friends, of course, I think it's rare that anyone sees it in themselves without any help at all) and seeking psychiatric help quickly - I'd seen one of my closest friends, a girl who really is like a sister to me, spiral down the slope of not getting the treatment she needed and she eventually ended up in a live-in psychiatric clinic for over a month with severe depression and anorexia nervosa. Having dealt with that and seen how much it destroyed both her and the people who cared about her, myself included, it wasn't a risk I was gonna take. She's doing incredibly well for herself nowadays, incidentally - lately the shoe's been on the other foot and it's been her supporting me through things, something which I can't really express how thankful I am for.
There's a very good short story (which also has a reading on YouTube) that my current psych showed me called I Had a Black Dog, written by a sufferer of depression and based on a quote from Winston Churchill in which he called his depression his "black dog". It's well-worth looking at, it gives an interesting perspective and some very accurate insight through the analogy.
So, to any other sufferers out there of not just depression but any mental illness, particularly those that have it much worse than I do as mine is thankfully not too severe, I've never had it go as far as serious suicidal thoughts - you're never alone. Keep the people who care about you close, don't give up or let your guard down and you can get through anything - it may feel like running against a brick wall at points, but stay strong and the wall will break before you do.
To close my thoughts out and absolutely at the risk of being a massive hipster by quoting song lyrics, Watch Me Rise by Have Heart, one of my favourite songs by one of my favourite bands, puts it quite nicely I think:
So I say to the slaves of depression
CARRY ON
And sing the sweet redeeming song
About living this life free and long