I've picked up a case of altoholism. I hardly log on to my main anymore, instead opting to enjoy the quick rewards of lowbie alt leveling.
Everything you do pretty much gives you immediate improvement at low levels. You can hear a ding in one night's play. You get new spells quickly. And as a friend said, players seem more likely to help each other.
I guess at higher levels we all get more self-sufficient. But at low levels killing a boar (think Bellygrub) can be a hard task. Riding or flying by someone is easier to keep going and forget you even saw them. Running by someone you're more apt to swing an axe to help them out, or throw a heal their way.
Soon I'll hit the point where the levels don't come as fast and the upgrades are few and far between. Traveling on foot from one inn to the next quest and back will start to get farther and farther apart as well (think STV). When the time comes, I'll just jump onto another alt.