Nothing-which is, indeed, a very random thing.
I am attracted to nothing and no one, yet there are times when I find myself aroused. No stimulus necessary. I find it a rather boring chore, so I will find ways to occupy my brain whilst my hand occupies my vagina. I once read the entire dictionary during a "session". It isn't that I find lexicon particularly arousing; it just so happened to be the only book in the house I hadn't read. If any were to walk in on me, they would certainly jump to that conclusion I'm sure.