The things I like best about my current and recent jobs, as a senior software engineer and software architect:

I get to work on software which will affect millions of people, in one way or another. I really like the idea of being useful.
The job requires constant learning. There's always something new to do, and new techniques to apply to solve problems better. Invent, invent, invent!
Programming and problem solving are incredibly fun to me, it provides excellent entertainment.
I get to work with a lot of very interesting, very smart people, and get to talk to them and hang out with them, and get paid for it.
Free candy and free pop. Bad for me, sure, but ever so yummy!
Software engineering: if you like it, I don't think there's a better job in the world!