There is a theme which I've noticed often underlying our discussions recently. It pervades the legendarium and, as it is "applicable", real life. This is the idea that the complete, or the perfect, is not achievable in the earthly world, and in fact, if one tries to achieve it, he is likely to lose much more than he stood to gain. Inversely, those people will be happiest who aim high but are satisfied with a proper amount of earthly perfection.
The obvious example, of course, and the one we discussed most in class, is that of the Numenoreans. They were given almost an earthly paradise by the Valar, but they tried to make this earthly paradise complete by gaining immortality; of course, we all know how that worked out for them.
This example's converse might be taken to be the Shire. This land, too, has natural beauty, and its people (usually) live in ease. However, they (unlike the Numenoreans) are content with their simple life, not seeking great wealth or immortality, and thus they are able to maintain it instead of falling into ruin.
Then there is Sauron, who wishes to conquer the entirety of Middle-earth (at least) and become the absolute tyrant of everywhere he can get his hands...er, Eye...on. Like the Numenoreans, he looks like he's getting quite close to his goal when he is thrown down and his power broken utterly.
Juxtaposed with this, of course, we have the example of Aragorn. He did great deeds, won back his kingdom, and lived a long life, but he did not seek to conquer more lands, and he chose his time of death when he was old and Eldarion was ready for the kingship, not sticking it out to the bitter end.
Now, what are we to make of this rule of Tolkien's world? Surely it is related to what we discussed in class about how something (or, to be specific, a created thing) could not exist entirely with no possibility for its opposite, such as how Light could not exist without the possibility of not-Light. It is not such a stretch to extend this to say that verygood-Numenor could not exist without the possibility of notsogood-Numenor; that is, a Numenor perfect on Earth with no faults could not exist.
And now we can begin to see the deep Christian foundations of Tolkien's literary philosophy. This "paradox of the complete" is straight out of Christian scripture and theology. The builders of the Tower of Babel, for example, strove to rival God, and were thereupon cursed with confusing languages. Even the problem of free will, which may seem a bit different, is actually closely related; for, were any person to have a will which is all good, then would he have no thought for evil, and his will would hardly be free at all.
The common thread here is simply this: that earthly perfection is unattainable, and, in fact, to attempt it is wrong. For God is the only perfect being, and to try to attain perfection without His guidance, as the Numenoreans did, is evil. "In you all things are finite...in the sense that you hold all things in your hand", as says Augustine*, and so we must accept that and not try to attain power through perverted means as did the Numenoreans and Sauron.