I suffered from anxiety and depression during my first year of college, and I adopted Arthur to keep me company. He was so active and he was just gorgeous. Over time he (and later his buddy, Merlin) among other things helped me overcome my problems, mostly because I was so overly-paranoid about them getting sick or hurt that I stopped worrying about myself!
My fish are my friends - they interact with me, they make faces at me. I spent several weeks creating 50 paintings and drawings of Arthur for an art project and in that time I came to know him very well. Whenever he sees me coming he comes up to the glass and waits until I put my face right at his level and then he'll swim around and wiggle.
They just have so much character!
Plus I love having something rewarding to devote time to.