Converting Users to Open Source- Why Do You Care?

mack knife asks: "Here's a question for Slashdot readers: Why do you care what web browser/email client/etc people use? What do you care if Firefox catches on or not? Why do some people feel the need to convert others to their pet applications? Personally, I am a convert to Firefox/Thunderbird, but I understand that many users are happy with their Microsoft products; I'll mention what I use and why, but I won't harangue them on their apps' shortcomings, nor will I try to push an unfamiliar open source app on someone who is more comfortable with a 'mainstream' product. Some open-source proponents can be quite obnoxious about this, and I'm interested to hear why it is taken so seriously."