As far as friendship goes, I also pulled out The Golden Rule, directing you to share your toys in the name of treating others like you would like to be treated. At the time, this seemed to be the most obvious rule or basic idea I could communicate in teaching you how to be a friend. You look out for the other guy's feelings, sometimes even at the expense of your own self-centered wishes.
The other half of the Golden Rule is what goes around comes around.
The idea being, that, if you continue to behave in such a manner, whether cruel or kind, the behavior you employ will boomerang back to meet you head-on. A repeated courtesy or offense will inevitably meet its match, because behavior always reaps consequences.
The Golden Rule as applied above, however, is missing a lot of groundwork in between. Certainly, it's a good start, and, in the course of daily living, an ever-present reminder to live less selfishly.
But why should you need a reminder to be kind to someone you are calling a friend? It seems to me that a genuine friendship needs no rule in the first place, for it maintains its inherent and understood honor, trust, loyalty, and respect from the very beginning.
There's more on friendship, but for now, I would suggest that The Golden Rule might be reworded: Do unto others as they are you in disguise.