If nobody shared files, there would be no files to dwnld. Sharing allows you to dwnld files from somebody across the other side of town or even another part of the world. Sharing is not compulsory. It's an individual's choice. However many users filter out those that don't share (nicknamed leechers or freeloaders b/c they don't share.) They may set their preferencing to only allow those people who share more than a certain no. of files to be able to dwnld from them. This no. might be 5, or 50 or 500 for example. ie: if you aren't sharing files, you simply wouldn't be able to connect to some people to dwnld from.

I forgot to mention, having people upload from you actually helps you to connect & connect to other people for better search results.

Could a person get in trouble for sharing? Well, not if they're sharing non-copyright material. And besides, with so many people using p2p, it's not the little people organisations like riaa are after. If you dwnld or/& share the most recent movies, etc. Then that might bring some attention to yourself. There's lots of non-copyright material being shared, but also a lot is.