Actually, I was thinking 'porn star' rather than prostitute, which makes the replies go on a different track than intended. While I'm sure the whole 'not as much fun when done for a job' thing still applies, it does so for different reasons.

I have actually had the opposite experience in my job as a software developer, though. I wanted to keep programming as a hobby, but eventually needed a career. I've found that I enjoy it even more than when it was just a hobby. There's more to it, I get to do it for longer, other people bring me ideas that I get to figure out how to implement... It's very rewarding.

I'm also odd in that I've enjoyed every job I've ever had, and miss some aspect of it, if not the whole job. I wouldn't give up my current job to go back, but they will always have a fond place in my memories. My experience is that most people can't say that about a single job, let alone all of them.