I disappointed my boyfriend by saying I "don't get anything out" of my relationship with his kids

I've been in my boyfriend's kids life for about 8 months now, so not a long time at all. They're good kid and he's an amazing father. I like hanging out with them (although I need breaks occasionally). They seem to like hanging out with me too, but at the end of the day I'm a well liked babysitter. Which is fine! It's just hard work taking care of two children, and maybe parents get more out of it because its their children and they have a biological bond (and also way more time to bond with their kids than 8 months). But I do my part, I help out, plan things, play, go out and do fun this together.

So I essentially told this to my boyfriend, that I don't get any joy out of step parenting that it's just work for me, not thinking it was big news, but I can tell he was disappointed. Obviously he doesn't blame me and he's not upset with me or anything. But I feel bad that I disappointed him. I feel guilty for not loving being a step parent, and not getting joy from step parenting.

Has anyone else been in a situation similar? How long did it take to really enjoy caring for your significant others kids?