I had one early reader of my books, a woman who knew me from my childhood, ask me why I wrote things so "dark." I have to admit it perplexed me because I never viewed my stories as "dark" exactly. Sure, my characters are dealing with dire situations, but the overall feel of my tales, then and now, centers around hope and perseverance no matter how dark the situations get for the characters.
I don't know why I write in the genres I do, I just know it's where my writing is strongest and where my inspiration takes off. Thrillers, horrors, and fantasies are also what I choose in movies, books, and other entertainment as well. Fantasy is not a genre I chose and it started gradually, but it is where my imagination took me. In my fantasy books, I stay grounded in reality but can stretch it and have fun with the "what if" possibilities.
I'm not a "dark" person at all. I like to laugh, have fun, and play in the sun. But when I write, I like to keep readers on the edges of their seats until the very end. And that, I'm told, is a very good thing.