Sharing files is what filesharing is all about. Search the forum for "Leechers" to see what I mean.

You are never forced to share. See the LW Library File menu for more info.

You'll also find people who say it's not good to automatically share every file downloaded (to cut down on fake files, spam and other malware). THe best way to do that is have an unshared download folder, and then move the good files to your shared folder once you've inspected them. This is how you can help the network prune the garbage.