Mitchell's sufferer "doesn't realize" that the toil is needless. In this conception, "at any moment he is permitted to step aside...."
Indeed, Homer's observation does not assert why Sisphyus pushes that boulder upward, merely that he does so ... again and again. Mitchell's modern conclusion about Sisyphus' "tragedy" is worth pondering.
S. Mitchell, "The Myth of Sisphyus," in Parables and Portraits (New York: Harper Perennial, 1994) = N. Kossman, ed., Gods and Mortals: modern poems on classical myths (New York: Oxford University Press, 2001), 165. — OGCMA1009NOTSisyphus_Mitchell