It varies for everyone, but for me what made fish fun...then turn into an addiction...was simply knowing more about them. When you have more details about their anatomy, environment, history, and everything else then they become fascinating creatures, and then you'll also know how to keep them as healthy and vibrant as possible, which will further add to your enjoyment. Many people like to experiment with different types of ecosystems, breed fish and observe all their growth stages, and more. Once you know what you're doing, the diversity possible in nearly any size of aquarium is truly astounding. As you can tell from some of my hubs (and more on the way), as well as 15 years of experience with freshwater aquatics, that was certainly enough to get me going.