Nietzsche has a quote that says "To those human beings who are of any concern to me I wish suffering, desolation, sickness, ill-treatment, indignities - I wish that they should not remain unfamiliar with profound self-contempt, the torture of self-mistrust, the wretchedness of the vanquished: I have no pity for them, because I wish them the only thing that can prove today whether one is worth anything or not - that one endures."
(The Will to Power, p 481)
Firstly I think it's important to say that Nietzsche didn’t write the Will to Power. He had started to write it but abandoned the work, instead choosing to write twilight of idols and the antichrist. His sister and a group of other authors took his unpublished manuscript after his death, and created the book. Their work is largely considered a massive distortion of Nietzsche.
Having said that though, I do believe that there is some truth to what he said. A lot of truth, in fact. He is not saying that suffering is a great thing, but rather enduring is a great thing, and that the more we can endure, the closer we are to being Supermen and women. The greatness of a person comes from how much suffering they are capable of enduring. He is implying that conflict and difficulty make us stronger and significantly more resistant to that level of suffering in the future. What doesn't kill us makes us stronger, therefore we should seek out difficulty in our lives so that we improve as people.
He is wishing a hard life on those he cares for so that they feel that their life is more meaningful and worth more. And that you need to have conflicts to overcome in order to grow and become a better person. Furthermore there's also the fact that without suffering you wouldn't know what peace and happiness even were. But, also that when the suffering does eventually end, you'll enjoy the peace that much more.
Hope you enjoyed and that this helps you
Please follow me if you'd enjoy similar content