You think because you have an intention to think. It comes from the idea that it is necessary to think, that there's a problem. If you watch, you'll see an intention to think, an impulse to think. Then comes the thought, and the thought may give rise to a feeling, which might give rise to another intention to think, and so on. You're not aware of that, so the thought appears as if it were coming by itself, and the feeling appears to be coming by itself, and so on.
