Nobody thinks of Codependency as a personality disorder, yet let's look a little closer at the dysfunctional set of human behaviors that are inherent within one's NEED to be needed.
Codependents are basically one-dimensional. They constantly yearn for acceptance and approval from others much like the Borderline does, and all their behaviors reflect this single-minded aim. Sadly, this makes them seem predictable, flat and even boring, to others.
Codependents have harshly judged and amputated all darker aspects out of their personality, from around 2 years old). They are only comfortable with and capable of GIVING, but not RECEIVING. They are incomplete, emotionally dissociated, partial-people, possessing only a slim fraction of facets that a normal/healthy human has in their personalty repertoire.
If someone reciprocates their attention and love, they feel a subtle, but nagging sense of obligation to give even more, to COMPENSATE another for what they've received from 'em, never once realizing the extent to which they already gave. Even a sincere compliment from anther can evoke feelings of awkwardness or discomfort.
Codependents are pathological people-pleasers. However much they give, it NEVER feels like "enough." Poor self-worth drives their compulsion (yes, I said COMPULSION) to assume they are lacking in worth or value, and must compensate for it 24/7. These people constantly try to make up for their imagined deficits, by over-giving so they can feel more deserving, lovable or generally better about themselves.
Codependency is a covert form of Narcissism. It always assumes to be more caring, more generous, more empowered, more strong, more patient, more giving, more wise or intelligent, and more adept than others.
The Narcissist seeks out opportunities both professionally and interpersonally, to demonstrate what they believe are the more powerful aspects they possess, but can ONLY exercise these with impaired others. These people have typically found work as psychotherapists, nurses, doctors, surgeons, social workers, life-coaches, etc.
Narcissism in the Codependent prevents them from choosing balanced, healthy, EQUAL partners, lovers or friends, which is why they are especially susceptible to seduction by Borderlines. Someone who does not require fixing or saving, doesn't inspire the Codependent Narcissist. He or she NEEDS to be needed, in order to assume the one-up position in all their relationship dynamics. This essentially assures that their fears surrounding full attachment, are mollified (if you date people who CANNOT attach, you're able to insulate yourself in a safe zone, emotionally).
As long as a lover is perceived as Less-Than, dysfunctional or impaired, the Codependent never has to step fully into loving, and has a built-in excuse NOT to bond or actually attach. It's actually a brilliant defense strategy, against one having to accommodate real closeness.
Both the Codependent and the Borderline share an equal measure of narcissism, which makes it impossible for either to endure a truly harmonious, mutually respectful and loving relationship bond. Each blames the other for their "deficits," but they share the same level of emotional development, and are equally incapable of attaching.
My online article, "HAVEN'T WE MET BEFORE? The Borderline/Narcissist Couple," goes into great depth about the compelling dynamic a Codependent and Borderline share. In truth, when you're in this type of relationship, it feels like you can't live WITH the partner you've chosen~ yet you can't live WITHOUT 'em, either.
In part, this is because you both have significant attachment fears and control issues, and each of you is vying for the one-up position in your intriguing but tormenting dance that has no happy ending.