I've seen some elements of why I dive in other folks postings, but here's my list:
1. The feeling of freedom when I'm underwater, suspended weightless, moving up, down, wherever just by thinking it (okay, a little fin work helps, but you know what I mean).
2. The sense of peacefulness and quiet: the one-atmosphere world does not enter my mind as I just wander around.
3. The views: from tiny critters to wall vistas, I remain in awe of what I see each time, even after 20 years of diving.
4. It is a great activity that I can share with my daughter.
5. The pursuit of corals and marine life sends me off to wonderful international locales.
6. Oh yeah, and IT'S FUN!