Many of the people who contribute to Linux do so because they feel it's "the right thing" in some sense (though that can mean a lot of things). There's a lot to be said for network effects increasing the value of Linux for everyone, but for a lot of people it's simply that they get their gratification from feeling like they've helped build something that other people enjoy and find useful. If more people use and enjoy what you've built it's simply more rewarding to do the work.
