Woldog is spot on.
I guess in my case it's pure fascination of looking into a world, where i myself couldn't survive.
Everytime i see a pond, river or lake i try to spot the fish that are in there.
I can only wonder about what's under the waterline.
At home i get a prime time seat and get lost in my fishes underwaterworld.
I love those endless observations most.
Plus the fact that the best fish interact with me.
I mean not only beg for food, but seem to recognize me.
Plus of course the handfeeding!
I don't mind if it's Guppies, Panchax, Carp, Oscar or Gar, getting a fish to trust you so much to be handfed, is really the bomb for me.