Sisyphus pushed a stone up a hill, and gravity rolled it back down.
If another person had been tasked with returning the boulder, Sisyphus might have come to view him as an enemy.
If instead, There were three stone movers, and the stones were numerous, (and perhaps smaller!) each would simply view the person before him as his stone supplier, and the person after him as his stone remover.
Each would see all as serving a role, but the task would still be pointless.