Patroclus was also Achilles mentee. Back then an older warrior would mentor younger ones into adulthood as a rite of passage. They would leave the polis and spend weeks in the wilderness learning how to hunt and how to fight. Once the mentee mastered the skills that would make him an adult, his mentor would gift him with weapons, armor, and a cup.
I strongly recommend Robert Graves' "The Greek Meeths". He collected all the classical myths and annotated them with anthropological and historical references. I read the Spanish translation 20 years ago, I am sure the English original is beautifully written.