I'll share a bit of wisdom my son shared when he was 4 or 5. In kindergarden they talked about what a friend is, and he said after some thinking: "A friend is a person you can disagree and fight with, and still play with later".
Edit: Not to belittle your son's wisdom, of course. I totally agree.
I think he mostly got it from my wife and I sitting down with our children after having a fight and telling them that we're still mom and dad, we've put it behind us, and it's okay not to get along all the time.