Glaucus and Diomedes refuse to fight because?

From what I understand about arete...the concept of greatness in all things...the Trojan Glaucus and Diomedes of the Greeks are meant to exemplify heroism and humanity.

When the two meet on the battlefield, Glaucus tells the story about how he descends from Bellerophon. When Diomedes realizes that his grandfather Oeneus hosted Bellerophon, their forefathers are friends of old.

Diomedes and Glaucus must also be friends. (Another concept stemming from arete.) They exchange gifts and decide not to fight each other.