Just as, in standing water, one sees his own face reflected back, so too is the heart of man reflected in the heart of another.
As I relate to my companion, so will he relate to me.
There are two aspects to this: the hidden feeling and the retrieval of what one thought was hidden.
Even when one cannot quite depict the nature of another's relationship to oneself, the automatic response is usually an accurate one.
It is not dissimilar from those responses in us which are not entirely under our control- like the contagiousness of yawns and laughter.
In certain people, who are especially sensitive to the social environment, there is a capacity for understanding one another which is beyond natural explanation.
As an example of such simple natural responsiveness, one may mention gratitude–a virtue which one does not learn from books, and which, if lacking, may be considered a sickness of soul.
In the same way, if one feels loved, it is almost impossible to remain indifferent; it arouses a similar feeling in response.
–Rabbi Adin Steinsaltz
From The Long Shorter Way, Chapter 46, by Rabbi Adin Steinsaltz