Hubris

Human nature is viewed differently, depending on where you live, culture, etc. The Epic of Gilgamesh says one thing about human nature: hubris is the downfall of any man.
In the beginning, Gilgamesh was portrayed as arrogant, boastful, and proud. He thought about nobody, save himself. He did what he wanted to do, uncaring of what his people needed him to do. He took the virgins; he took the sons; he had no regrets. Gilgamesh had the perfect body, the perfect strength, the perfect everything. Even he forgot he was only two thirds god, not three thirds god. He forgot about his one third of mortal human. Gilgamesh had forgotten about his humanity, thus allowing hubris to overwhelm him.
Then Enkidu enters the scene. Enkidu is the polar opposite of Gilgamesh – he runs with the animals, he looks like an animal, and he helps the animals. He is very in touch with his mortality. He does not think of himself as a god, or even close to it. The book mentions that after the harlot had cleaned Enkidu up, he was like a god. Yet Enkidu never boasted the hubris that brought Gilgamesh down.
When these two met, they fought, and became friends. These polar opposites became brothers. This is the beginning of Gilgamesh’s downfall. The notion of Enkidu not living for eternity never enters Gilgamesh’s thoughts. He assumes, incorrectly, that they will both live forever. But humanity takes its course, and Enkidu dies, leaving Gilgamesh broken hearted. This death forces Gilgamesh to look at his own humanity, and he suddenly realizes he will die, too. His hubris of believing he was pure god (or merely forgetting he was human) brought his downfall.