Mammele: I think the source may be “Michtav Me’Eliyahu” by Rabbi Eliyahu Dessler who was, among other accomplishments, the mashgiach ruchani of the Ponevezh yeshiva in Israel.
He analyzes the powerful bond between parent (mother) and child and devises the theory that it results from the effort the mother devotes in caring for and nurturing the child. The more effort one invests on behalf of another, the greater the resulting feelings of love and compassion generated in the benefactor. The way I understand it, by investing one’s self in the success and well-being of another gives one a personal interest – or mission – in the the success of the beneficiary. It’s not limited to mother and child; it’s a fundamental Human trait that can be applied in any situation.
This novel and insightful notion runs contrary to conventional thinking that love is something that can be bought through favors and gifts (in other words, the notion that one can trigger reciprocal feelings of love in the other person. Rather, we can only generate such feelings in ourselves; we don’t control the feelings of others. The more conventional thinking is the mistake too people make in their desperate quest to be loved by the object of their desires.
To me, this explains many ideas, from HaShem’s love of mankind, peoples’ extraordinary, selfless, charitable efforts on behalf of complete strangers all the way even down to, le’havdil elef havdalos, the bond of people with their pets.