The Epic Of Gilgamesh
Theme of Friendship
Gilgamesh and Enkidu starts off as enemies but their bond help them become the best of friends.
Enkidu is the "rational" part of the friendship; once he is gone it is not necessarily his death, but his absence and inability to counsel Gilgamesh, that leads Gilgamesh to go off on such an irrational quest.
When Gilgamesh goes from lamenting the death of his friend to lamenting his own future death, his feelings for his friend don't diminish. They are so close that when Enkidu dies, Gilgamesh is basically losing a part of himself