Did you choose your genre, or did your genre choose you?

A little of both. I tend to write in genres and age categories that I prefer to read. Sometimes, though, I get an idea that doesn't fit what I've written before or even what I've read a lot of. If the latter, I then go and read a bunch in that genre/age category.

I've written YA, MG, chapter books, and adult
I've written fantasy of different stripes, SF, contemporary (poorly), and even literary (with a hint of fantasy)

I will probably never write romance or erotica because I have no interest in reading those genres. I like mysteries, but more of the classic Agatha Christie kind, so I probably won't be writing that genre. I prefer thrillers as movies. I occasionally have a craving for historical, but would never be confident enough writing it. And so on.

More often than not, I read and write MG and YA SFF. These are the genres and age groups I grew up reading, and I never grew out of them. I've found that if I try to write contemporary, I almost always throw something fantastic in by the end (It's a reality show about dancing, but, hey, there's a ghost somewhere in there). I'm pretty diverse in fantasy, but for SF, I so far have stuck to a single 'verse.