I used to skydive a few years ago.
Some people are unable to understand why someone would want to skydive. Skydivers call these people "whuffos", which comes from the question "Whuffo you jump out of them perfeckly-good airplanes?"
Other activities also have their whuffos as well. So how do you explain to a whuffo why you'd ever want to hike 2000+ miles straight?
The only answers I can come up with are that I love hiking and backpacking, and that thru-hiking the A.T. is something I've always wanted to do.
So essentially I'm left with "explaining" that I want to thru-hike the AT because, um, I want to.