It reminds me of a phrase that I think comes from Tolkien somewhere: the "long defeat", referring, I think, to life generally; at least to a particular aspect where the Christian and Germanic worldviews match up well. In the Germanic worldview, where lof was the greatest thing one could hope for, everything was in more or less an ultimate process of dying and going away: lif is læne! Even the gods would loose the ultimate battle and be consumed in the end. Correspondingly, in the Christian worldview, the "ruler of this world" was Satan, and our ultimate victory would really only come in the next. See, e.g., Matt 4: the temptation of Jesus, where Satan is in a position to offer all the kingdoms of the world to Jesus. And in the anonymous epistle to Diognetus a few centuries later, the author notes that the Christian is ultimately an outsider even among his own people, since his real citizenship is to the kingdom of heaven.

So much for Tolkien's negativity. His optimism comes, surely, from what he saw Christianity adding to the end of the story: the ultimate victory of the afterlife, and whatever little foreshadowings of that we are able to glimpse here (One of these, e.g., that he explicitly mentions, is the Catholic Eucharist, since it both harkens back to the Crucifixion, and forward to the victory of heaven.)

Aside from these negative and positive sides to intellectual ideas, sometimes (for some people anyway) there is on a more primeval level a paradoxical kind of comfort that comes from being able to be melancholy about things, and having that melancholy confirmed by something you care a lot about: for Tolkien, that was both his Faith and the heroic Germanic worldview. For example, the Western custom of designing crosses with the corpus (as opposed to the crosses of the east) is said to stem from medieval Europe's horrific experience of the Black Plague. The Suffering Christ was a great source of comfort and even solidarity. I think when someone has undergone great suffering, they don't just want to hear "It'll all be better"; sometimes one simply wants to know that there is another that knows suffering and understands. Tolkien had a similar experience I believe from World War I.

Hmm. Interesting: this aspect of humanity must explain why there seem to be so many religions with stories of a divine-type figure suffering. Christian doctrines on suffering alone (and maybe how much Tolkien would have known/participated in them) could potentially be an entirely different, huge (and deep) thread.