The original story was intended as a depiction of a punishment. The spin of imagining him happy was Albert Camus’s contribution. Although it wasn’t quite about finding joy in a never-ending repetitive process per se, but more about finding joy in the absence of meaning. The boulder process represents the individual’s repeated, doomed efforts to find meaning, and the rolling back down is the realization that despite their efforts, there was still no meaning to be had. Yet, the human cannot resist trying again, which is clearly absurd - hence why this is called “The Absurd”. The main idea is that the struggle itself towards meaning, although absurd, is enough to make someone happy, even if meaning does not exist. I would not want to conflate that sentiment with acceptance of repetitive, dull processes in real life. Nietzsche would have a lot to say about that, and not coincidentally, Nietzsche and Camus have a lot of overlapping thinking. If you like this stuff, I’d recommend reading Camus’s The Stranger, Nietzsche’s Thus Spoke Zarathustra, and Camus’s Myth of Sisyphus.