Sisyphus' punishment was to roll a rock up a hill forever, but then why couldn't he just drop it? Why couldn't he get it to the top?
Homer describes the stone as being so heavy that, while Sisyphus could carry the stone part of the way up the hill, he would get so tired doing so that he would then drop the stone before he got to the top.
Aye, and I saw Sisyphus in violent torment, seeking to raise a monstrous stone with both his hands.  Verily he would brace himself with hands and feet, and thrust the stone toward the crest of a hill, but as often as he was about to heave it over the top, the weight would turn it back, and then down again to the plain would come rolling the ruthless stone. But he would strain again and thrust it back, and the sweat  flowed down from his limbs, and dust rose up from his head.
I've seen a variety of explanations as to "why couldn't he just drop it," but I don't have a reputable source for these explanations as of right now (presumably because these explanations may not be supported by primary sources). One popular claim is that Sisyphus was promised immortality if he rolled the boulder to the top of the hill.
However, the simplest explanation (and the one best supported by the primary sources) is that Sisyphus had to push the boulder up the hill because that was his punishment.
But Sisyphus is punished in Hades by rolling a stone with his hands and head in the effort to heave it over the top; but push it as he will, it rebounds backward.
The image of Sisyphus pushing the bolder up the hill is used by philosophers like Camus as a metaphor for life. Although this is a modern interpretation of an ancient story, it might answer some of the questions people have about it.