The friend who is a helpmate,
the friend in happiness and woe,
the friend who gives good counsel, the friend who sympathizes too -
these four as friends the wise behold
and cherish them devotedly
as does a mother her own child.
Sigalovada Sutta, Digha Nikaya
Buddha Quotes: Page 5
I've just had reason to look up the sayings of Prince Siddhartha, and this one seems apt, given the reasons.
What I find interesting about quotes like the ones on the link that quote was taken from is how much people knew about each others' behavior back then. At least, the wise ones did. Many times, that wisdom was dressed up in religious trappings, but the basic understanding of our nature was clearly there. Why and how all that behavior comes about is still a mystery, and social scientists and biologists will probably spend decades more figuring that out, but how all that neural activity plays out has been known and recorded for a long time.
As Buddha might have put it:
See the false as false,
The true as true.
Look into your heart.
Follow your nature.
Buddha Quotes: Page 2
Understanding your own mind is a good way to understand others'.