In his essay on Smith of Wootton Major, J.R.R. Tolkien speaks of the reluctance of the Faery folk to allow Men use of their power (or magic). He writes this:
The love of Faery is the love of love: a relationship towards all things, animate and inanimate, which includes love and respect, and removes or modifies the spirit of possession and domination. Without it even plain “Utility” will in fact become less useful; or will turn to ruthlessness and lead only to mere power, ultimately destructive. For this reason the Elvenfolk are chary of giving to any human person possession of any device of their own which is endowed with Elvish power called by Men by many names, such as magic. Most Men will certainly misuse it as a mere instrument for their own personal power and success. All men will tend to cling to it as a personal possession.1
Man, fallen, desires power and knowledge, and not merely power and knowledge, but more power and knowledge than his neighbor. The hearts of Men are weak if they are not strengthened with humility, and they easily fall prey to sweet-tonged promises of dominion. This is why, in The Lord of the Rings, Sauron is able to draw nine mortal men into his grasp so effortlessly: he wooed them with promises of power and they saw only their gain and were blind to their undoing.
Elf-magic is powerful and perilous: powerful because it is carved by hands that are purer and stronger than those of Men; perilous because Men are weaker than those hands and unable to comprehend the potency of the raw beauty the immortal Elves can handle. Unless corrupted, Elves create for beauty’s sake. They do not consider the dominion their magic may give them over others. Men see only the power of Elf-magic and the authority it may give them, not what beauty it may give to others.
Man, in his pride, does not see his own folly. It is this blindness that causes Boromir to covet the One Ring in The Lord of the Rings. He sees its power and the chance of victory it will give his country. The fact that the Ring is wholly evil and exceeds his ability to control has no impact on his desire.
Elves are immortal and have seen many lives of Men and know their fallen, prideful nature. However, Tolkien says in the same essay that the Elven folk “have a permanent love for [Men] in general” and, though not bound by moral obligation, “they do from time to time try to assist them” and “avert evil from them.”2 The people of Faery foresee what might happen to the world if they let Men go their way. For Elves love beauty and decline in its absence and know that Men also need some beauty for their survival. Thus they shield Men from a measure of destruction by denying them the use of their magic.3
1J.R.R. Tolkien, Smith of Wootton Major essay, found in Smith of Wootton Major “Extended Edition,” edited by Verlyn Flieger
3This is not so in all cases. There are times in ancient lore when the Elven folk deem a Man worthy of their confidence.