The boulder pushing punishment is iconic. But I think more people should know the reason Sisyphus was punished to begin with, which was for cheating death, twice.
The first time he cheated death, Sisyphus had just angered Zeus by revealing the location of the Asopid Aegina whom Zeus abducted. Which is super valid, fuck Zeus.
Sisyphus knew that Zeus would send the god of death Thanatos after him, so he prepared a trap and trapped Thanatos in the chains meant for him.
After that, nothing on Earth was able to die so long as Thanatos was in chains. Which meant no animals could be sacrificed to the gods. This angered the gods, who made Sisyphus' life so miserable with pain and illness that he would beg for death. And so he released Thanatos.
But then came the second time Sisyphus cheated death. As he was dying, he asked his wife to dump his naked corpse in the middle of the public square. Denied a proper burial, his soul ended up on the far side of the river Styx, unable to cross.
He complained to Hades and Persephone about how his wife disrespected him, and begged them to let him return briefly to the world of the living to scold her and make her bury him properly. They agreed, and Sisyphus returned to life. He then embraced his wife, and refused to return to the Underworld.
It's only when he finally died of old age that he was sent to Tartarus and punished with the boulder.
I don't remember where I've seen it, but I like the interpretation that Sisyphus doesn't have to push the boulder. He can choose to stay in Tartarus and rest. But he was promised that if he managed to push the boulder to the top of the mountain, he'll ascend to Elysium.
And Sisyphus, in his stubbornness and cleverness, refuses to give up on a challenge.
One must indeed imagine Sisyphus happy, planning and scheming about how he'll cheat the gods next.