"Like water – face to face; so also is the heart – man to man". (Proverbs 27:19)
King Solomon equates water's reflective qualities to the human heart, or rather, to human attitude. Just as my smile or frown [my face] is duplicated in the water's surface, so too, my attitude is reflected in another person's expressions: the other person embodies the reflective quality of water. If, for example, I am generous to another, he too will be compelled to be generous to me. The question is, how does this dynamic work? I understand the physical laws that generate reflections, but when it comes to human nature, why are attitudes also "reflective"?
What is a reflective attitude? I believe there are a few components. First, humans must draw inspiration from others: we are designed that we empathize, naturally. If you are generous to me, it is natural that I reflect this feeling of generosity. But getting passed the "feeling", I also display or reflect this generosity back towards you.
I think this second step is born out of a need for companionship...we reciprocate to sustain this good relationship. Juts like everything else we do is inescapably selfish, here too, we enjoy the good others perform for us, and wish it to continue. We are recognized, validated, and naturally enjoy others that make us feel this way.
However, reciprocating the good we receive can be elevated beyond simple insurance that others continue to favor us. We can recognize and follow God's will that others exist; for God created them too. Assisting that person to live happily, by reciprocating his or her good, is the higher form of reflecting good will. And an even higher form, would be initiating good will towards others, not merely responding. For in this case, we are not performing good due to a received benefit, but due to a recognition that kindness is God's will, regardless of our personal interests. This explains why the Torah commands us to assist even those who have wronged us, if their animal's load is overburdening it. (Exod. 23:5)
 By selfish, I mean to say that in all human actions, we act for the self. Even when giving charity, we do so for our own good, or a self image, or we wish to follow God's laws. We are doing so as we see this is a good for ourselves somewhow. Selfish does not have to mean that we are not giving. Just as boyish refers to the qualities of a boy, "selfish" is meant here to refer to qualities or interests of the self.