How would jobs work under anarchy if they are voluntary?

Wouldn't people just back out when they get offered to do the tough jobs in society? And then there would be no people to do those jobs. And i myself don't really believe that all humans are non-selfish by nature so how would this work?

This is where anarchism loses me, people are selfish by nature and so they will back out when they "have" to do jobs that are hard (since it will be voluntary to do those jobs under anarchism because anarchism is voluntary).

This has been giving me a massive headache. I keep coming back to this and i can't figure it out no matter what i do.