Adventure and Exploration
Fitness
Fun
Personal challenge
Social reasons
Stress relief
Utility purposes (ie. commuting)

All of the above, but I might add that I love bikes: fast ones, slow ones, new ones. old ones, big ones, small ones, road bikes, mountain bikes, hardtails, full suspension, tandems, hybrids, classics, cruisers, electrics, touring, cruising, trail riding, technical riding, road riding, beach riding, MUP riding,
you get the picture.