Trust, Loyalty and Vulnerability (or Everyone has their Kryptonite and their Lex Luthor)
In many epics, the hero has to struggle with the question of who he can trust. In "The Song of the Cid", the Cid entrusts his daughters to the king who are very dear to him. They function as his weak spot. Nevertheless, he lets the king decide who they marry and does not question his choice at all. The king makes a very poor choice and the daughter's of the Cid end up half dead on the side of a road abandoned and beaten by they treacherous husbands. Yet, even when that happens, the Cid does not blame the king for choosing such unworthy husbands. He even lets the king decide about the same issue for a second time. His loyalty almost seems to be blind faith in his leader.
In "Das Nibelungenlied", Siegfried tells his wife Kriemhild about his vulnerable spot because he is certain that she is never going to tell anybody about this. Kriemhild on the other hand trusts her uncle Hagen, when he is asking her about Siegfried's weak spot. She is naive and tells him about the place between his shoulders where Siegfried can be wounded. Hagen lets her mark the place, attacks Siegfried from behind and kills him.
Almost every single epic hero has a weak spot: Achilles has his heel and Siegfried the spot between his shoulders. Arguably, the daughters are the Cid's vulnerable spot and Gilgamesh's is certainly Enkidu. It is like Superman and the Kryptonite. The only way of wounding the hero or of defeating him is attack his weak spot. Therefore, the hero has a tough decision to make. He either has to keep his vulnerability secret or he has to tell their secret to a trustworthy person. But if he trusts somebody he can, of course, be betrayed.