One way to explain it is to ask if your phone company (which is your ISP) should be allowed to block Skype/Facetime. After all, if you're skyping (or facetiming), then you're not paying them for a phone call! (Or should Skype be allowed pay your phone company to block Facetime (or vice versa)). Surely it's in the phone company's interest to not help their compeditor. (If your internet is from your cable TV provider, ask if they should be allowed block Netflix).
Examples like that are easy to show the downsides of net neutrality.
I like thinking of it like other utilities, such as water and electricity. Imagine if your electric company could block power to one of your devices because it's a competing device. Or if it could stop you from using water for drinking to promote some drink they have.
The data going through should be just as blind as water through pipes or electricity through cables.
Examples like that are easy to show the downsides of net neutrality.