In The Unbearable Lightness of Being, it is said that:
In the world of eternal return the weight of unbearable responsibility lies heavy on every move we make. That is why Nietzsche called the idea of eternal return the heaviest of burdens (das schwerste Gewicht).
What is so heavy about living a life over and over again?
Wouldn't it be boring, maybe by realizing through seeing the process like watching a film, even watching yourself live repeatedly? Wouldn't you become indifferent and uncaring to the process?