Even if you've never heard this term, I bet you can think of someone who fits the description. Pia Mellody is the expert in this field and the following theories come from her book, Facing Love Addiction.
Possibly the most significant characteristic of love addiction is that we assign too much time and value to another person. Love Addicts focus almost completely on the person to whom they are addicted. At the beginning this relationship feels fantastic. The Love Addict feels special, it's a kind of high. They experience relief from the pain of feeling empty, of being alone. The connection is usually intense and finally gives meaning and vitality to the Love Addict's life.
It is believed that people fall into love addiction because of the unhealed pain from childhood abandonment, and the feeling that they cannot be safe in the world without having somebody else hold them up. They cling to a delusional belief that the other party has the power to take care of them, affirm them, and somehow make them complete. Love Addicts usually didn't have enough appropriate bonding with their caregivers, and probably experienced moderate to serious abandonment or neglect in childhood. Because they weren't nurtured for who they were, they have trouble being or liking their natural selves.
Eventually, as Love Addicts try harder and harder to manipulate the other person to live up to the mental image they have created -someone who will care for and love them the way they long to be cared for and loved- they experience repeated disappointments, because no one can satisfy these insatiable desires. Love Addicts begin to retaliate with toxic fighting against what they interpret as a willful failure to love on the part of the other party.
The irony is that while Love Addicts want to avoid being left and want to be connected to someone in a secure way, the close, demanding connection they try to establish is actually enmeshment rather than healthy intimacy - which they also fear, at least unconsciously. This denied fear also comes from the childhood experience of either physical or emotional abandonment. Love Addicts did not experience enough intimacy from their abandoning caregivers to know how to be intimate in a healthy way.
This is the cycle of the Love Addict.
Love Addicts are usually drawn to Love Avoidants, who tend to avoid commitment and healthy intimacy. This makes Love Addicts desperate and needy for their partners' affection, always demanding more out of them. And since they cannot tolerate the thought of being alone, they stay in the relationship, becoming more and more angry. They can't leave, because they fear abandonment; but they can't be comfortable staying, because their desire to be rescued, cared for, and protected isn't being satisfied.
Love Avoidants, meanwhile, fear intimacy because they believe that they will be drained, engulfed, and controlled by it. This is because in their families, the child was expected to nourish the parent. These enmeshed children get drained dry and used by Mom's or Dad's need for companionship, attention, and love. This experience of childhood enmeshment created a deeply ingrained conviction that more intimacy will bring more misery.
It's complicated, though. Children who have been enmeshed develop the idea that taking care of needy people brings them self-worth. They believe that taking care of needy people is their job. When they stay in relationships, it is often out of duty and to avoid guilt, not love. This is the only way they know how to have relationships.
Paradoxically, while Love Avoidants actively avoid intimacy, they also fear being left. This fear is usually unconscious. Because they were valued for their caregiving, they derived their self-worth from it. Although they resent this role, it's the way they know to receive attention and love. They worry that if they don't allow themselves to be engulfed by their partner, they'll be worthless.
So Love Avoidants have the same two fears as Love Addicts: intimacy and being left. The difference is that what is conscious for one is unconscious for the other. Love Addicts have a strong fear of abandonment and an unconscious fear of intimacy, which causes them unconsciously to pick someone who can't be intimate. Love Avoidants have a strong fear of intimacy, and yet also deep underlying fear of being left. This keeps them on the front edge in relationships, where, for part of the time, they can feel powerful by meeting someone else's needs without being engulfed.