For me, it is the satisfaction of having my "dream build" and never being a peasant ever again.
Instead of leveling my Main to 34, I plan on leveling my shielder alt to 33 as well simply to have another dream build.
I find it rather enjoyable, as I view it as a "reward" and not "oh damn, ran out of things to do." It helps that there are new maps, and new players or old changing players to keep things interesting and fresh. As a bonus for having an archer main, every patch also changes the mechanics enough where my playstyle radically changes too, forcing new tricks.
Then again, I'm the guy with two and a half dozen active alts, so....
*shrugs*