It used to happen to me very often when I played video games--it's pretty understandable if you think about it. When you play a game in waking life, you are so concentrated on it that you almost put yourself into the world of the game anyway. Dreams just give your mind the power to make that more literal.

For a while when I was younger, every time I got into a car, I would suddenly be driving a car like I would drive them in Grand Theft Auto. (When I was dreaming, of course)