When men master-bate

What is it that inspires a man to masturbate?
Does he play in his mind, hot sex with someone?
Or was there a picture of a woman giving a man a bj?
Maybe a scene from a porn video?

As he wraps his hand around his cock
Does he picture a hottie licking and sucking it?
Using his thumb as he rubs the head, imagining it is her tongue
He tightens the grip a bit more
Does he imagine it is her tight little pussy wrapped around it, nice and snug?

So what is it that gets men to CUM when he takes matters in his own hands??