Many reasons really, in no particular order:
As a throwback to childhood.
To enjoy places I miss going too fast in the car and are just too slow to see walking.
To get away from it all, it clears my mind while I'm out and about.
To share time with friends. Or, just as much time alone.
To get fit. If you knew me that would be a shock to hear, but it does get addictive.
Something to look forward too during the working day.
And the bikes themselves.
Relaxing. Thrilling. Hard work. Rewarding. Overall enjoyable.