Why, Ilúvatar, why???

"Then Ilúvatar spake, and he said 'Behold I love the world, and it is a mansion for Elves and Men. But the Elves shall be the fairest of earthly creatures, and they shall have and shall conceive more beauty than all my children, and they shall have greater bliss in this world. But to Men I will give a new gift.'

Therefore he willed that the hearts of Men should seek beyond the world and find no rest therein; but they should have a virtue to fashion their life, amid the powers and chances of the world, beyond the Music of the Ainur, which is as fate to all things else. And of their operation everything should be, in shape and deed, completed, and the world fulfilled unto the last and smallest. Lo! even we, Elves, have found to our sorrow that Men have a strange power for good or ill, and for turning things aside from the purpose of Valar or of Elves; so that it is said among us that Fate is not master of the children of Men yet are they blind, and their joy is small, which should be great." 

Morgoth's Ring, p. 21