The Return of the King
Brain Snacks: Tasty Tidbits of Knowledge
In a letter to his son Christopher in 1944, Tolkien confirmed that the whole orc thing is a more general metaphor for the extreme evil of which men are capable: "Urukhai is only a figure of speech. There are no genuine Uruks, that is folk made bad by the intention of their maker; and not many who are so corrupted as to be irredeemable (though I fear it must be admitted that there are human creatures that seem irredeemable short of a special miracle)." (source, pg. 90.) While Tolkien may assign the different peoples of Middle-earth moral values (as in, elves are super-good and orcs are super-evil), in the end, both "elf" and "orc" is just a way of talking about extremely good or extremely bad human beings. There's a little bit of elf and a little bit of orc in us all.
Still, even though the elves are generally Good People in Middle-earth, it's important to remember that they are not perfect. Consider this fairly scathing assessment in another letter, written to Naomi Mitchison in 1954:
But the Elves are not wholly good or in the right. Not so much because they had flirted with Sauron; as because with or without his assistance they were 'embalmers.' They wanted to have their cake and eat it: to live in the mortal historical Middle-earth because they had become fond of it (and perhaps because they there had the advantages of a superior caste), and so tried to stop its change and history, stop its growth, keep it as a pleasaunce, even largely a desert, where they could be 'artists' — and they were overburdened with sadness and nostalgic regret. (Source, pg. 197.)
In other words, the elves can be self-indulgent and even a bit spoiled. They are hanging around in Middle-earth because they are the best people there. Back home in Elvenhome, they might have more competition. No wonder they're so sorry to leave!
A last word about Tolkien's views of his own creations: Tolkien insists that "Middle-earth" is not supposed to be a fantasy world outside of our reality. These characters aren't aliens or anything. And they aren't created out of nowhere. He writes:
"Middle-earth," by the way, is not a name of a never-never land without relation to the world we live in (like the Mercury of Eddison). It is just a use of Middle English middel-erde (or erthe), altered from Old English Middangeard: the name for the inhabited lands of Men 'between the seas.' And though I have not attempted to relate the shape of the mountains and land-masses to what geologists may say or surmise about the nearer past, imaginatively this 'history' is supposed to take place in a period of the actual Old World of this planet. (Source, pg. 220.)
Wait, what? We were on our earth this whole time?