People who love MMOs tend to cite the fun of social, shared play as the reason. It's more fun to play together than alone. However, at the higher levels, when it becomes necessary to group just to stand a chance of completing a raid or instance, it becomes difficult to find a group of players of the same level as you -- unless you're already at the highest level you can attain. One thing to be said about reaching the ceiling is that you can stop worrying about leveling and concentrate on some good, solid 'endgame' content with a bunch of like-minded players.
This begs the question: is all game content merely there to help you get to the endgame, at which time the fun truly begins? When you're bored with your top-level character, and you roll a new one, do you grit your teeth and sigh expressively through the lower-level content until you're back up on top? Once you've reached the heights, is the rest of the game still fun?