I have quite a few personal projects of my own—my software, my websites, and my World of Warcraft mods. From time to time I get offers from people who really want to work with me to improve them. Sometimes these offers even come in the form of complete prototypes. I usually turn these down.
Even after all this time, I still feel a little bad for doing it, but I almost always do. These are my personal projects, and I need to keep them personal, completely controlled and owned and managed and written by me. If I don't, and they become some multi-person effort, then sure, they are more likely to thrive and live on and be useful to the public, but I'm also a little selfish—they become more and more like work, and less and less like something I work on in my leisure time. The main reason I have all of these little side projects is that while I like to build things, so much that I do it for a living, it's simply a lot more fun to do it with the full control and lack of overhead that comes with being the only guy in charge. If I start having to run ideas by another person, or stay on top of what other people are doing, or Heaven forbid ask people for permission before I start doing things, then they're no longer fun, and if they're no longer fun, then I shouldn't be doing them in my spare time, because I do plenty of software development work already that's not always "fun." It's not that I don't enjoy my job, it's... that I don't want more of my life to be like my job than already is. I have to draw a line somewhere.
Some random person from the internet started sending me modified versions of one of my World of Warcraft mods a couple months ago, wanting me to add those features into my official version, and wanting feedback and direction. He must have spent quite a bit of time on all of his modifications. I appreciated his attitude, wanting to help make my project better, he annoyed me. It wasn't his fault, but I don't think it was mine either. Time I spend working on those projects is time I'm not spending on other things (like playing games), and I have plenty of other things I'd rather be doing than code reviews and testing what he wrote. He didn't write things in the way that I would have written them, and he added features that worked well for him, but wouldn't have been too useful for anyone else. Basically, after all this time he spent working on his own, I just had to refuse pretty much everything he gave me. I didn't feel great about doing it, but I can't be selfless all the time. At work, I'm one voice out of several, and it's the job of those several to keep me in check and make sure that I'm making the right decisions. That makes sense at work, but not in my hobby.