What is the lesson that sal gives to Jennings in the diner ?

Sal gives a lesson on friendship and helps him understand that it's okay to have relationships, that even when someone goes away..... it doesn't mean that the relationship wasn't special. He told him that hurting when someone leaves is natural..... but that it's the amount of hurting defines just how special that person really was to you. Thus, Sal teaches him that it's alright to open up and allow people into your life. Sam also talks him into going home.