‘You may learn much of them,’ said Sador, and he sighed. ‘They are a fair folk and wonderful, and they have a power over the hearts of Men. And yet I think sometimes that it might have been better if we had never met them, but had walked in lowlier ways. For already they are ancient in knowledge; and they are proud and enduring. In their light we are dimmed, or we burn with too quick a flame, and the weight of our doom lies the heavier on us.’
‘But my father loves them,’ said Túrin, ‘and he is not happy without them. He says that we have learned nearly all that we know from them, and have been made a nobler people; and he says that the Men that have lately come over the Mountains are hardly better than Orcs.’
‘That is true,’ answered Sador; ‘true at least of some of us. But the up-climbing is painful, and from high places it is easy to fall low.
- The Children of Húrin: The Childhood of Túrin
As a shadow Melkor did not then conceive himself. For in his beginning he loved and desired light, and the form that he took was exceedingly bright; and he said in his heart: ‘On such brightness as I am the Children shall hardly endure to look; therefore to know of aught else or beyond or even to strain their small minds to conceive of it would not be for their good.’ But the lesser brightness that stands before the greater becomes a darkness. And Melkor was jealous, therefore, of all other brightnesses, and wished to take all light unto himself. Therefore Ilúvatar, at the entering in of the Valar into Eä, added a theme to the Great Song which was not in it at the first Singing, and he called one of the Ainur to him. Now this was that Spirit which afterwards became Varda (and taking female form became the spouse of Manwë). To Varda Ilúvatar said: ‘I will give unto thee a parting gift. Thou shalt take into Eä a light that is holy, coming new from Me, unsullied by the thought and lust of Melkor, and with thee it shall enter into Eä, and be in Eä, but not of Eä.’ Wherefore Varda is the most holy and revered of all the Valar, and those that name the light of Varda name the love of Eä that Eru has, and they are afraid, less only to name the One. Nonetheless this gift of Ilúvatar to the Valar has its own peril, as have all his free gifts: which is in the end no more than to say that they play a part in the Great Tale so that it may be complete; for without peril they would be without power, and the giving would be void.
- Morgoth's Ring: Myths Transformed
It's said that "for it seems to the Elves that Men resemble Melkor most of all the Ainur."( Of The Beginning of Days), and I think this jealousy of the greater brightness is definitely a noteworthy example. For the Edain, it's greater knowledge, strength, closeness to divinity, and most importantly the immortality of the Eldar, which they longed for. The fear of death was the consequence of their first fall and the root of the second fall of Númenor. For Melkor, it's the power and majesty of all the other Valar, the light bestowed to Varda and above all, the Imperishable Flame of Eru that kindled sentient beings, which he'd sought for but couldn't possess. As Sador said, climbing up for the greater light is painful because Men and Melkor could never obtain the one thing they wanted most. The more they had, the more they were dissatisfied with what they've had, and so the further they went, the harder they fell.