Friendship is an important motivation for many characters in the Iliad; at times, it can make them act in ways that you wouldn't expect, given their other loyalties. For example, when the Trojan Glaukos and the Achaian Diomedes discover that their ancestors were bound by ties of "guest-friendship" or xenia (see our summary of Book 6 for details), they decide that they can't kill each other. Instead, they exchange armor. Similarly, Achilleus's extremely powerful friendship (some would say love) for Patroklos makes him forget his rage at Agamemnon and join the battle on the Achaian side. Depictions of non-romantic friendship between members of the opposite sex (Patroklos and Briseis, and arguably Hektor and Helen) round out the Iliad's nuanced portrayal of this important human emotion.
The Iliad portrays friendship as an especially enduring connection.
Friendship is less important in the Iliad than anger and pride.