I can't really say what it is about playing guitar.
It's fun, to be able to make music is an awesome thing. It's cool, there are millions of guitar players in the world but it's still kind of an elite club. It's stress relieving like Benson said. It's self gratifying, you feel good about your self when you accomplish what you've been working on. It's makes you smarter (music itself anyway) kids who play music are academically smarter than other kids.
I'm in construction and on a daily basis if I whack,cut,pinch,smash, or scrape my hand, the first thing I think is "can I still play".
Lots of us have asked ourselves why we didn't start playing sooner.