The key to understanding codependence is understanding what happens to a child whose needs are not met. Imagine a family in which there is alcoholism, or serious illness, or something else that is taking up lots of the family’s effort and emotion. A young child in this family would find that when he or she has needs, most of the time all the attention is taken up elsewhere, and the child’s needs are neglected. The child may even get punished for being demanding, or difficult, or whiny. A child in this kind of family deeply absorbs the lesson that his needs are a problem for the people he loves, and that it is better not to have those needs. A child internalizes this, and will try very hard, now and in future relationships, not to be needy.

The child in this circumstance typically learns a second powerful lesson:    She learns that it doesn’t work well to have needs, but it seems to work well to take care of the needs of others. In this kind of household there are usually lots of unmet needs, and a perceptive child will learn that she can be much appreciated if she is trying to meet them.  This is the pattern described by Alice Miller in her excellent book, The Drama of the Gifted Child.  ( Alice Miller is a German psychoanalyst, and the book was written for therapists, but has since been widely read by the general public.)

As a child settles into this pattern, of trying not to be needy, but to meet the needs of others, the behavior is endlessly rewarded and reinforced by everyone nearby.  It begins to feel natural and secure to take care of others and be valued for that.  At the same time the child feels deeply flawed and insecure about the times when she feels needy rather than helpful.  Because the family didn't deal well with her needy, human side, it becomes a habit to avoid showing that side to anyone, and to constantly seem helpful and self-sufficient instead.  It comes to feel quite natural to have a particular kind of relationship, in which she takes care of others and they do not take care of her, typically because she doesn't show much sign of needing care. 

This is a very seductive pattern, because a person can spend an entire lifetime taking care of others, and being appreciated for it.  The problem is that the person sets himself up to be used, and to be not really known by the people he is close to.  That in itself is a lonely experience.  To make matters worse, this pattern often leads a person to chose partners poorly.  If someone is good at taking care of people and asking for little in return, he is likely to wind up with someone needs taking care of and offers little in return, rather than winding up with a mature and healthy person.    This is a pattern that was often noticed in the partners of chronic alcoholics. 

Essentially what happens is that a person develops a deep insecurity about whether he will be loved by someone who doesn't "need" him.  People get caught in patterns of dating or marrying "needy" people rather than mature and healthy people.  This is because it feels more secure to be needed than to believe that a mature, healthy person who doesn't "need" you will stay around simply because they love you.