I work alone on my own projects and usually decline offers to collaborate. This is largely out of personal preference, and I think some people prefer to work alone and some enjoy working on a team more, but there are also practical considerations: as a programmer I don't want to end up just providing free programming for someone else's creative vision, it's hard to avoid differences in opinion on direction and features (unless you're totally in sync like the Cohen brothers), if no one else is getting paid then contribution and commitment will vary, and if the project is successful, you may find people have different expectations on how rewards and IP ownership. Working on my own forces me to keep my ambitions pragmatic, learn about areas outside my core expertise from art to marketing to web design to legal stuff (even if I'm not proficient at them and have to contract out), work at a comfortable and enjoyable pace, and I own what I create. I have to work on teams anyway whenever I have contract or salaried work, so working on my own projects is a nice break from that!