The third essential for emotional stability is intimacy. More than an outlet for our sexual energy (but vital for that as well), we are a specie that craves a relationship with at least one other person that goes much deeper than friendship.
In the Bible the term often used for intimacy was "knowing". We crave a relationship in which we are known, where we are freer to be ourselves, where we can be seen both psychologically and physically naked and know that we are loved.
Because life is inherently risky, our survival is significantly enhanced if there are two of us. If something happens to me, you will be there to catch me, just as I will be there for you. And if I want to celebrate my success, share my joys, it's not much of a celebration if I am all alone.
It is in intimacy that I get to express my sexual nature and learn to see the world through the eyes of the opposite sex in a way that allows me the depth of vision that only comes from binocular vision, and can never be dreamed of in an ocular world.
A loving, prolonged and sexual relationship with a person different from ourselves broadens our understanding of life and is a vital part of a balanced life.