If you work out on a weight machine that has a limit–where you have to push the bar until it stops–you're far more likely to to hit that limit than if you had left it to your own initiative to figure out how far is far enough.
People enjoy going to the max (or in the case of Spinal Tap, a little farther than max, to 11). But if there is no max, no limit, it's much easier to satisfy yourself and declare that you've done enough.
If you want your best users to do more, one way to do it is to announce the most they can do. While this may dissuade a few people from pushing ever farther, it will in fact motivate a large number of people to up their game.
"The maximum number of times a week you can dine here is three."
"The maximum bonus paid is $100k."
"The maximum number of tweets per day is 30."