My top reasons are:
1) It's a healthy obsession. I cycled a bit 20 something years ago, very casual recreational. Got more serious about it last year. At almost 60 I'm in the best physical condition I have been in many years, if not ever. Still have a few pounds to loose but I don't take any medications, have low BP, and I sleep better then before I was riding.
2) Started group riding last year, social group doing no drop rides. The group rides have been the biggest driver to improving my cycling skills and abilities.
3) The thrill of reaching personal cycling milestones, I did my first century last month, may do another next week. Two years ago a 15 mile ride on a flat MUP was a big ride for me. Many of the hills I rode last year seemed to have gotten flatter this year.
4) Cycling clears my head, reduces stress, it's fun and gives me a better appreciation of the land and features that I miss while driving a car.