People work harder when there is someone looking over their shoulder.
I'm going to be completely honest with you here. When I work alone I spend a considerable amount of time surfing the web, reading email, twittering (you can follow me at @mendelt) getting coffee and talking to coworkers. This means I'm spending less time working but it also means I'm constantly interrupting myself during the time I am doing work making me even less productive.
Now we don't tell our managers this because it wouldn't do anyone much good, the problem is a bit more complex than just people slacking off. What I've observed is most people have a hard time pacing themselves. We do really focused work for five minutes and then take a break. Unfortunately breaks have a habit of taking more time than they should and productivity goes out the window.
To solve this problem we need a way to maintain a sustainable pace. Pair programming is a way to do just this. Forcing yourself to explain what you do to your pair is a great way to maintain a sustainable pace and work a bit harder.