Every glance at the world, to explain which is the task of the philosopher, confirms and proves that will to live, far from being an arbitrary hypostasis or an empty word, is the only true expression of its inmost nature. Every thing presses and strives towards existence, if possible organised existence, i.e., life, and after that to the highest possible grade of it. In animal nature it then becomes apparent that will to live is the keynote of its being, its one unchangeable and unconditioned quality. Let any one consider this universal desire for life, let him see the infinite willingness, facility, and exuberance with which the will to live presses impetuously into existence under a million forms everywhere and at every moment, by means of fructification and of germs, nay, when these are wanting, by means of generatio aequivoca, seizing every opportunity, eagerly grasping for itself every material capable of life : and then again let him cast a glance at its fearful alarm and wild rebellion when in any particular phenomenon it must pass out of existence; especially when this takes place with distinct consciousness. Then it is precisely the same as if in this single phenomenon the whole world would be annihilated for ever, and the whole being of this threatened living thing is at once transformed into the most desperate struggle against death and resistance to it.Nietzsche, at various times, seemingly rejects the idea that we can know the inmost nature of the world and insists that this inmost nature is will to power. (People who read Nietzsche tend to believe that he contains multitudes or else that we should carefully exclude remarks he probably did not mean so that a consistent Nietzsche emerges.)
Bernard Reginster argues that Nietzsche wants to improve on Schopenhauer's theory. Schopenhauer, he says, identifies suffering with resistance to the will, sees suffering as an inescapable feature of life, and thinks the best we can do is resign ourselves to it. He thinks that Nietzsche, in contrast, wills (or advocates the willing of) suffering so that the will has some resistance to overcome. So both see suffering as inevitable but Schopenhauer pessimistically regards this as bad and unavoidable, while Nietzsche optimistically regards it as good and embraceable. He sounds a a bit like Camus on Sisyphus.
Reginster knows much more about Schopenhauer and Nietzsche than I do, so I'm probably wrong, but I wonder whether maybe Nietzsche is misreading Schopenhauer if this is what he really thinks. For one thing, willing endless suffering does not sound like the most optimistic thing one could do. For another, as Iris Murdoch has pointed out, Schopenhauer isn't really that pessimistic. He recommends not mere resignation but an enlightened love of the world. Life, after all, will go on after my death, and so will the matter I am made of. Some people find no comfort in this idea, but Schopenhauer praises dust:
Oh! Do you know this dust then? Do you know what it is and what it can do? Learn to know it before you despise it. This matter, now lying there as dust and ashes, will soon form into crystals when dissolved in water. It will shine as metal; it will then emit electric sparks; It will, indeed, of its own accord, form itself into plant and animal; and from its mysterious womb it will develop that life, about the loss of which you in your narrowness of mind are so nervous and anxious.(WWR, Volume 2, p. 472)
Since we are mostly water I imagine we will evaporate and then fall as rain, like the newlyweds and others on the train at the end of Larkin's "The Whitsun Weddings." Perhaps most importantly, Schopenhauer writes that it would be just as accurate to call the world embodied music as embodied will. This is not a resigned, pessimistic idea. Nor does it suggest that we should be too literal about interpreting his ideas about the will to live. Suffering, finally, belongs to the rather illusory world of maya. Enlightenment releases us from it.
In fact, since suffering helps us find enlightenment, Schopenhauer claims that it is desirable. He gives the example of the satisfaction people take in hard physical work. At this point, the contrast with Nietzsche's position is hard for me to see.