When they came back, the gods are mad at them. The gods determine Enkidu must die as punishment for their foolishness. The gods taking Enkidu away from Gilgamesh is their way of telling him to grow up.
In the early books, Homer is trying to compare the actions of Achilles to those of a child. After Agamemnon takes Achilles prize, the Briseis, from him, Agamemnon vows that he will not fight in the Trojan War anymore. Then he runs to the beach to cry to his mother. Achilles then begs his mother to go to Zeus and ask him to punish the Greeks until they give Achilles the respect he deserves. This is showing his selfishness in thinking that the gods should castigate an entire army just for his self-esteem.
Achilles then goes and tries to convince the rest of the Greek army to sail home and abandon the war with him. He no longer is crying like a child, but being rebellious and insulting Agamemnon. This shows his transition into adolescence. The three ambassadors Odysseus, Ajax, and Phoenix come to Achilles and try to get him to release his anger and join them back on the battlefield. Phoenix takes on the fatherly role when he calls Achilles his son, and Achilles rejects the appeals of the older men just as expected of an adolescent to reject the advice of a parent.
After Enkidu dies, Gilgamesh starts fearing death and goes in search of Uta-napishti and eternal life. This is Gilgamesh's mid-life crisis. He realizes death will come looking for him so he tries to find a way to escape it. Uta-napishti counsels Gilgamesh and tells him that he can't defeat sleep, so there is no way he can defeat death. Uta-napishti informs Gilgamesh that there is a plant that will rejuvenate his youth, but it is at the bottom of the ocean. Just like forty year old men today will go buy a new sports car when their hair starts turning grey, Gilgamesh risks diving to the bottom of the ocean for this temporary youthfulness.
