The Bitcoin system turns out to be socially useful and valuable, so node operators will feel like they're making a beneficial contribution to the world through their efforts (similar to various projects where people voluntarily donate their computing power for good causes).

In this case, I think simple altruism may be enough to keep the network running properly.

This is very attractive from a libertarian point of view if we can explain it correctly. Though I'm better with code than with words.

Satoshi Nakamoto