It almost feels random for me. I always wanted an aquarium, but can't easily have one for various reason (cats and space, mostly). I always loved flowing water--I like to hike to waterfalls, for example. So I kept a fountain for about 10 years, but I kept having to fill it, as in three+ times a day, due to wind and it became something that wasn't worth the hassle. So I got the idea for a fountain in the ground, which evolved in my mind into a waterfall, and after a read a few books, I determined that it would be best to have a "receptacle" for this water. I wasn't originally planning on having fish. But as I started digging, I kept reading and seeking out new information, which led me to conclude that I should hedge my bets and make it big enough to accommodate fish. I have very heavy clay hear, and mosquitos started breeding in the hole I was digging, so then fish came to seem essential. Then I also realized this could be my way to finally have a sort of aquarium, too.
One thing led to another as I kept digging and building and reading (and reading some more), and I ended up with a waterfall -> stream -> pond, with a another waterfall going directly into the pond, which is about 12'x16'x20 inches or so, maybe bigger, and deeper in spots. This totally transformed my yard (the waterfall/stream part extends a further 15-20 feet from the pond), so I had to alter the landscaping to suit the pond. That's what I've been doing this year (I started building my water feature almost exactly one year ago), as well as adding a third waterfall to accommodate additional filtration. Fish multiply.
I would say it's more work than I anticipated, but I kept expanding my project's scope, so that's kind of moving the goal posts. Now, I couldn't imagine not having fish and the sound of streams and waterfalls in my yard. My neighbors all love it, too, because I live in a city, and though our neighborhood is quiet, there is always ambient noise, which the waterfalls and stream help to mask. I am so glad I got into this hobby. The primary two drawbacks are cost--this has been a very expensive proposition, though the costs will go down dramatically now that all the building is about done--and it makes travelling a bit more difficult. Ponds demand attention. Creating a real ecosystem is both awesome but also a responsibility. I am not one of the experts, but I love having my pond.