"Eternal Recurrence" as Zarathustra's Most Abysmal Thought

Nietzsche thinks Thus Spoke Zarathustra is his greatest book and the doctrine of eternal recurrence is the “most Abysmal Thought” of this book. Why has this doctrine such importance in the philosophy of Nietzsche? Although there are differences between eternal recurrence in The Gay Science to Zarathustra, but always remains a few characteristics. First, Nietzsche's eternal recurrence is a measure for revaluation of all values. The eternal recurrence is against the ressentiment and the ascetic ideal. Second, although Nietzsche in The will to power, sometimes has some sentences that imply to cosmological interpretation of the doctrine of eternal recurrence, but most interpreters have emphasized the practical implications of this doctrine, although about the modality of these implications there are disagreements among them.
The problem that arises is that if eternal recurrence requires that we say yes to everything, including unfavorable affairs, and even the "amor fati" implies to love pains, how to say yes to pains is compatible with to say yes to life? The answers to this question clarify new dimensions of Nietzsche's thought and the doctrine of eternal recurrence. This doctrine causes "creativity", "choice", "decision" and "activity". Eternal recurrence completes and leads nihilism to its peak and this is also desirable for Nietzsche.