The version I like the best is that Sisyphus keeps pushing the boulder voluntarily, because he's too proud to admit that, despite all his cleverness, there's something he can't do. (Specifically, get the boulder to stay at the top of the mountain).
My favorite version is similar. Each day he tries to push the boulder a little higher, and as the boulder starts to slide back, he mentally notes his improvement before racing the boulder down to the bottom with a smile on his face.
Because he gets a little stronger and a little more skilled every day, and he knows that one day he'll succeed.
Here's the new thread for posting quotes, with the usual rules: