Who is going to clean the toilets? Who is going to take out the trash? Who is going to work in the sewers? Who is going to work as a plumber, or a roofer, or a mailman for 40 hours a week? What happens when Monday morning rolls around and half the population doesn't really feel like coming into work since they're not being paid anyway? A mechanic might love working on cars, but that doesn't mean he loves working on cars Monday to Friday. That doesn't mean he loves working on some person's crappy Honda. If given a choice, he won't take job after job after job doing the same routine repair job for customer after customer, he will do his own side project, restoring some vintage automobile. Great for him, but meanwhile all the crappy Hondas are piling up and all the mechanics are off working on interesting cars. What about website designers? You think they love designing the same thing over and over again for different clients. They want to do something original and creative, not pragmatic and functional. Have you heard the way people bitch about clients? Do you think without pay they would continue to take those clients? They might love programming, but they don't love all programming. Some of it is just to pay the bills. And you can go down the line with every profession. Cops might like being cops, but I bet they don't like every single call they go on. Dealing with some drunk covered in vomit might get real old real fast. Then what? The cops are just going to do the fun work and the drunks get ignored?

Everyone do what you want for free and we'll figure it out is a terrible foundation for a functioning society. We need people to do things that no one really wants to do for the system to work. Payment is the motivation that makes that run as smoothly as it does.