The reasons people stop flying is surely as varied as the reasons people start.
Personally, I wanted to fly since as young as I can remember (3 I think). I used to read all the books on the RAF and wartime pilots as a kid. Later I would read Richard Bach, St Exupery books etc. I had a dream, as you could say.
Sadly medical reasons stopped me from RAF flying duties, and even a commercial career in flying.
The dream was still there as soon as I could afford (just) to take my PPL though. I passed in 1988 and had 12 great years spending everything I had (after living expenses) on flying, gaining ratings and flying wherever I was based in the world. I had a 1/3 share in a beautiful Stampe, and rented complex tourers. I could not contemplate life without flying.
Suddenly, at a medical renewal, I learned that my condition (unchanged since birth) would no longer be allowed for a CAA certificate. So overnight, I stopped.
I was so bitter and twisted for a while towards the CAA, but eventually just moved on with life.
Nearly 9 years on and I need to fly again. The JAA medical limitations still prevent me from gaining their "honoured" standard (on a ridiculous technicality). Thank goodness the FAA use common sense. I am nearly through an FAA IR course. (Still want a Stampe again though).
If it's in your blood, you'll always look up to the sky. You can walk away, but you'll be back.