The way I like to think of it is that non-copyleft licences are like giving everyone freedom by saying there are no laws - suddenly, you can do anything, and the government can't stop you! However, other people can also do anything and the government can't stop them, either, and that includes using a big net to catch other people and make them their slaves. The people caught in the nets aren't going to feel very free anymore, and it's not unreasonable to think that a lot of people will end up caught in nets.

Copyleft licences are like saying there are no laws except you're not allowed to do anything that would restrict someone else's freedom. In theory, that's only going to inconvenience you if you were going to do something bad, and leaves most people much freer.

The idea is basically that you shouldn't be able to restrict anyone else's freedom to modify the software they use, and if you're going to, you don't get to base your software on things made by people who didn't.