This is not presented as a cosmological theory but as an existential test. How would you live if you knew that every choice, every moment, would recur eternally? Would you embrace your life with such passion that you would will its eternal repetition?
The eternal return becomes a measure of life-affirmation. To say "yes" to eternal recurrence is to embrace existence fully, to live in such a way that you would choose it again and again. It challenges us to live without regret, to create a life worthy of eternal repetition.
Source
Thus Spoke Zarathustra - Friedrich Nietzsche