I've always had it. My parents both have varying degrees of depression (as an idea - my mum takes 80mg of citalopram a day, I take 20mg - which is considered the normal amount ppl with depression take).
For me though it really flared up nearly four years ago, when I lived in Melbourne, away from my now wife working at a place that had good 'foot soldiers', but a toxic culture perpetrated by the 'highers up'. My gf also worked in such a place, and got to the point where she left her job and I had to support her. I guess the only thing keeping me sane was - although I was lonely - I was saving lots of money, but without it I just spat at, thought 'what's the point?' and quit my job, and almost quit the world.
It took a while, but for me meds got the ball rolling. There's a thread of mine that is essentially a play-by-play of my recovery so I won't post it again here.
But you're right, it can hit anyone. Look at all of the wealthy people, rock stars etc who take their lives. Shit's a double edged sword - you can have everything in the world, but when you do have everything, it's so easy to just go "what now?" and go into that spiral.