I've answered this one a thousand times around these parts already, but I'll say it again. I just love being able to conceive of something in the morning, create it in the afternoon, and by evening see people all around the world use something that was just a glimmer in my eye 12 hours beforehand.
I also love the psychology of UI/UX. Color theory, how little design elements and placements of things on the page can affect emotions, and how you can get someone to do something without them realizing that you're the one behind them doing it. "Making" them want to buy your product, for example.