Remember the experiment where they put a wire in the pleasure centre of a rat's brain and it would repeatedly press the on-button until it died of starvation?
Turns out that it wasn't the pleasure centre, but something more akin to curiosity, anticipation or foraging. It's the same bit of the brain that goes "Ooh, an interesting link, I shall click on it.", or "If I slay one more Orc then maybe I'll find something cool", or "I shall hit refresh one more time on LJ to see if anyone has posted in the last 3.5 seconds."
So if you've ever wondered where an hour went while you link-hopped on wikipedia, or why you haven't seen your partner in the week since they bought the latest World of Warcraft expansion, now you know :->