I loved Lego and Playmobile when I was a child and DF feels a bit like playing Lego or Playmobil with dwarfs living in my constructions.
Another reason is, that I am rather new to it (february I think) and haven't seen it all yet. I haven't built on an aquifer, in a desert, at an oceanshore, haven't tried out water- or magmapumps, got the first time slaughtered by hitting "hidden fun stuff". And I'd like to try out a megaproject or two (there's a 3dwarf program to visualize your fortress in 3d).
I have fun to make everybody, including the nobles, ecstatic and build fancy appartments for them, and try to manufacture good stuff for a character in adventure mode.
For me, part of the fun is the graphics. With the ASCII I can imagine the stuff that happens and the stuff I build the way I like them. But I think, when I've done all the things that I myself wanted to do, I might lose interest, because the game itself delivers no real aim (yet).
Btw: Goats are killers. Ask my hunter.
