It's so hard to say what "the" reason is because there are so many.
Everyone's had dreams about flying - We get to live them. For centuries, people have looked skyward to the birds and wondered what it'd be like to fly. Many people dedicated their lives to pursuing the dream of flight, and many lost their lives in that pursuit. Now that we can do it, and do it quite safely, it seems crazy that not everybody does it!
I love learning. There's never a shortage of learning opportunities in aviation, whether it be the intricacies of regulations, the stats on the latest sleek bird, how to identify various aircraft, all the history behind aviation... And it's pretty cool to be able to go out to Fisk during Airventure to talk with (during the airshow) and observe the controllers, and when one says "Anyone know what that one is?" be able to spit out "Anderson-Greenwood AG-14!" I'll never tire of learning more about aviation and passing what I've learned on to others.
I love operating machinery. I have an engineering degree, but I've been obsessed with machines since I was little - I was always excited about getting to use the lawn mower, then the lawn tractor, then the snowblower, etc... Then I drove cars, then buses, then trucks. Airplanes are complex machines, and I like to not only fly them, but also manage all the systems.
I like mental challenges. It's fun for me to calculate descent profiles in my head, for example. No shortage of potential in aviation here either.
I love variety - The spice of life. There are so many vastly different things we can do in aviation, and I've loved all the ones I've tried. Gliders, seaplanes, aerobatics, pumpkin bombing, jets, turboprops, fly-ins, $100 hamburgers, business travel, pleasure travel, etc. etc.
Speaking of travel - That's another thing I really enjoy. And there's no better way to do it than GA. Yes, there may be times I can't go, but for most of those times I wouldn't even be going if I had to drive or fly on the airlines. For those "gotta be there" situations, I like to leave early enough that I could use an alternate method of travel if I had to. In 10 years and 1200 hours, I don't think I've had to yet. I even decided to fly somewhere I was originally going to drive due to icy roads (sky had cleared up after the storm but the roads were still a mess). I figure that in the Mooney, I can beat the airlines anywhere east of the Rockies, and I can have way more fun doing it. Now that I'm married, it's even cheaper sometimes (plane costs the same, airlines cost 2x as much). When there's kids in the picture, that'll tilt things even more in favor of GA.
And, of course, the wonderful feeling of freedom and pure joy that we all get from being up in the sky, away from normal life and its problems, with the view of the gods. It simply can't be beat.