Viva Carlos really does put out some great questions! This one in particular motivated me, since I get this question sometimes. Why DO I continue to ride?
Well, it's both incredibly complex and very simple.
Simply put: I love to ride. I love my horse; I love horses in general. I love the feeling of partnership and zen that we get when we work hard together and are both on the same page. I love the rhythm of it, I love being outside, and I love just the feeling of riding. I joke that I keep my sanity in my saddles, and if I stay away from them too long I start to slip further away from sanity. :)
I also just love that everything else slips away when I ride. Work, life stress, pain (mostly; I couldn't fight through broken bones or recently torn cartilage), worry, etc. It all just goes away and my attention is on my horse, my ride, my friends, etc.
There's also, like SMTT mentioned, the social aspect. I love the people I've met through horses. We all tend to have some things in common: willingness to work (I don't know many hugely high-priced riders who don't work for their rides), appreciation for these wonderful animals, and at least in the eventing community, a love of the outdoors and being a little scrappy. :) I love the supportiveness of the eventing community, too. Need a girth/whip/shirt? Just ask! Sticky places on course? Of course you'll tell someone else before they leave the start box. Et cetera.
I love the impetus to get out and exercise, too. To echo SMTT again, if I slack off, Apollo tells on me. :)
The complicated part comes from the fact that I've been hurt, moderately severely, more than once, and I have taken my own sweet time really learning how to ride effectively ... and yet I stick with it. Somehow, I haven't managed to give it up in 30 years ... I had a long break after college, but I rode every chance I got and still loved horses and riding. Honestly, I think that one is either a horse person or not, and you can't really get away from it if you're a horse person. We ride because, on some level, we must. I am pretty sure that if the zombie apocalypse came and horses weren't zombified, I would still manage to find a horse to ride. :) I might not be great, but I would find one.
I'd bet my readers are the same.