We are "social animals", to a certain extent, and all need the 'external' feedback for a sense of self. Such things as job/community status and significant, intimate relationships come under this extrinsic pathway. However, we really need a solid intrinsic pathway which is our independent, internal self-valuation of feelings of self worth. This internalised self-view needs to be positively nurtured in childhood to a "good enough" degree.
So, if we do not have this "good enough" perspective we can become dependant on others' feedback. In order to elicit this feedback we can do a variety of behaviours to "gear" it - mainly either:
It works like this, at a subconscious level: I imagine that... "If I can really nurture, care for, love and do things for other people, then I can try and convince myself I'm OK and deserve to exist" (compliance) and/or "If I can get that person to think, feel and act like I do then I must be OK and have valid thoughts, feelings and actions" (control).
Of course, there is no getting away from the "strings attached" when these reactions are in place. Almost any response becomes coloured by this needy/driven state. E.g.: "Advice giving" would be to make one feel important and keep people dependent on us; "the need to be needed" can create frenzied activity; always comparing oneself or keeping score with others or being fearful of "what will 'N' think?"
The "others" to whom this activity is directed may begin to feel swamped or controlled and start to rebel or avoid.
The codependent person begins to assume responsibility for the other (not just toward the other) often to the exclusion of meeting his or her own needs. This creates frustration, resentment and depression, compulsions and denial. The person very dependent of the external others will be constantly on the lookout and may often perceive a situation way out of proportion. They will often be "supersensitive" and become even more frantic and unrealistic about what can be controlled by force of will and invest extreme amounts of energy into "fixing" up the others/outside world often beyond the fixable. Hence, they may put up with abuse when they need to let go.
All of this begins to blur the boundaries between the self and others, and an over personalisation will occur, there will be a significant loss of objectivity - everything will have a personal, emotional meaning.
Others will feel intruded upon, and misunderstood, and the codependent person will always feel misunderstood after a while.
Relationships become intense and enmeshed and individuality is hard to tolerate, people talk of their "soul mate" being found for the twentieth time and are continually searching and restless and feeling incomplete or lost. Healthy intimacy tolerates individuality and can share meaningfully without living in each other's pocket.
Perspectives become out of balance. Extremes of thinking occur. E.g. black & white, all-or-nothing rationales are common.
The whole reaction/counteraction in mind and action becomes "self fulfilling" and quite driven. Co-dependency is sometimes called "the other addiction".
But it can be treated. Even being aware of the relationship between one's self esteem and one's actions is a major advance. Establishing balanced behaviours/thoughts, and working on intrinsic self worth can all be accomplished and assisted therapeutically. There is also a 12-step self-help organisation called CODA (Codependents Anonymous on 0055 620 40).