I ride because I love the being on the trails and I wanted some interesting exercise. Running is work for me - it doesn't engage me enough to stay interesting. Riding engages me completely - requires strength, endurance, agility, and focus. It lets me get the woohoo feeling of being 10 again and accomplishing something new and cool. I love the sights, the different character and terrain of different trails.
It burns lots of stress and keeps me from being a hair trigger. 😀
I like tinkering with stuff (incorrigible hobbyist) and biking allows lots of tinkering.
Its funny that I started for exercise - now I try to jog and lift between rides to be a better rider.
I like the comaraderie (albeit smartass, self-righteous, and sarcastic) of fellow riders. I fit right in.
It is the coolest, most all encompassing, engrossing thing I've ever done.