1. Just cuz its fun to sail.
For me: making a good long run in a steady force 4 or 5, averaging 8 or
more knots for a couple of hours. Just whizzing along the waves, even
though you can do that easier in a small hobie.
2. Seeing new places that can only be seen by boat is fun.
Main reason are islands. You never go there unless by boat. Other shores
are easier to reach by plane.
3. Companionship
And loneliness! Alone at night with all other crew sleeping, in a gentle
breeze with the stars above...
4. Thrill seeking
See 1. Unless thrill is sailing in the South China sea, hoping not to be
pirated :-)
5. An obsession with problem solving (my personal fav)
As little as possible, but can be demanding and rewarding.
6. Choose yer own reasons.
Animal life! Only albatross you'll ever encounter will probably be on a
boat. Same with dolphins, whale, etc.
And one mo I forget the huge cost of running a boat very easily once
I'm under sail and I can pretend sailing is for free! ;-) And in
opposition to powerboating, it probably is.

Victor