You understand that you enjoy living in a system that you can trust. You understand that being able to trust the system means that the system has to trust you. You value a working system more than a petty dollar or some other short-sighted piece of self-interest, because you get more benefit from having the system work in your favor than taking a little for yourself but eroding/ruining the system in the same go. If most people do the right thing, the system works and everyone benefits.

Simple as that! It's the opposite of the tragedy of the commons.

In non-utopian countries, you have to make an advance deposit into the system and may never do enough to make it universally trusted. People may never stop jaywalking, or taking a free coffee/waffle/etc. from the unmanned stand, or cheating your taxes with family income and business expenses. But I'm doing well for myself, so what am I really losing by doing the right thing? Life already put me ahead. I'd rather contribute a small part to a working system I can trust, than to grab another small pie just for myself.