A long, long time ago, Men awoke in the Wide World. They awoke alone, and journeyed through the plains, seeking out more peoples. They went through the woods, and found the Dark Elves. The Dark Elves could tell that they were newcomers to the world, and being a good people, taught them language, craft, and hunting.
The lives of Men were calm and content. They found more of their kind, and settled in villages with them. Having woken under the Sun, they loved its light, and so would work and play under it. When it set and the Moon rose, they would go home and sleep.
Then, one day, a dark and terrible figure appeared over the hills. Some Men fled and hid at the sight of it, but the rest were frozen with fear, too afraid to do anything but quaver where they stood.
The figure raised a hand. "Fear not," it said. "I do not wish to harm you."
The chief took a hesitant step forward. "Who are you?"
"My name is Melkor, The Strongest and The Mightiest of the Valar! I come to bring you news."
"What sort of news?"
"I have come from the Far West, and soon, the Noldor will be here as well."
"Who are the Noldor?"
"The Noldor are Elves, like those you have already met. But they are different; they lived in the Far West, the Undying Lands. The land is a paradise, with beautiful things that last forever. The Noldor learned great craft from my fellow Valar there, and so can create many wondrous things. They can make lights that can last through day and night, stones that let them see all around the world, and more. And they are coming to the Wide World now because they want to claim it for themselves."
The gathered people began to glance at each other. "How do we get to the Undying Lands? We want to live there too and learn from the Valar." the chief said.
"Alas, you cannot. My brothers and sisters would not allow it."
"But why not?" Everyone cried out in dismay.
"Because they do not think you worthy. Only Elves are worthy of their grace in their eyes, because of the eternal life they granted them. But they will not give you aid, for they see you as weak and mortal, doomed to die."
The Men began to mutter amongst themselves, envy growing in their hearts. Melkor smiled, for that was exactly what he wanted. "I however, think otherwise. I think you do have worth. But you need help, which I am happy to provide you with. Swear yourselves to me, and I promise you, you will have all the world has to offer, and more."
And so, the Men swore themselves to Melkor, and he in turn told them many more lies. From that point on, Men would never be content with what they had, always wanting more and more, and they would have the drive to get what they wanted, oftentimes with no heed to the consequences of their actions. It was because of that oath that Men are like what they are today, with all the good and ill that entails.