For me, system design is intrinsically enjoyable, and learning new systems is enjoyable as well, so I like seeing and doing lots of original system design.
For Laser Kittens specifically I had a particular dynamic that I wanted to model -- a kitten building up uncontrollable energy until they just explode, with unpredictable results, but with the ability to shape and channel it as they mature. I didn't see any existing systems with anything similar, so rather than spending a lot of time searching for the perfect thing to hack, I went ahead and designed my own.
-Stentor