Do you have to share your files when using LimeWire?

No, you can select not to share your files with other users. Go to your library, right click on the files you don't want to share, and select "Stop Sharing File."

But if everyone in Limewire wont share files, gnutella community dies, and limewire would be useless, as there will be NO files to download because nobody shares.

Think about this: you want to download, so means you expected others to share, yet you refused to share yourself. So selfish!