Bonding is a biological and emotional process that makes people more important to each other over time. Unlike love, trust, or attraction, bonding is not something that can be lost. It is cumulative and only gets greater, never smaller. Bonding grows with spending time together, living together, eating together, making love together, having children together, and being together during stress or difficulty. Bad times bond people as strongly as good times, perhaps more so.
Bonding is in part why it is harder to leave an abusive relationship the longer it continues. Bonding makes it hard to enforce boundaries, because it is much harder to keep away from people to whom we have bonded. In leaving a long relationship, it is not always useful to judge the correctness of the decision by how hard it is, because it will always be hard.
Moreover, experiencing together extreme situations and extreme feelings tends to bond people in a special way.. Trauma bonding, a term developed by Patrick Carnes, is the misuse of fear, excitement, sexual feelings, and sexual physiology to entangle another person. Many primary aggressors tend toward extreme behavior and risk taking, and trauma bonding is a factor in their relationships.
Strangely, growing up in an unsafe home makes later unsafe situations have more holding power. This has a biological basis beyond any cognitive learning. It is trauma in one's history that makes for trauma bonding. Because trauma (and developmental trauma or early relational trauma is epidemic) cause numbing around many aspects of intimacy, traumatized people often respond positively to a dangerous person or situation because it makes them feel. It is neither rational nor irrational. If survivors can come to see that part of the attraction is, while very unwanted, a natural process, they may be able to understand those feelings and manage the situation more intentionally.
An excellent book on the effects of trauma (and repair) is The Body Keeps the Score: Brain, Mind, and Body in the Healing of Trauma by Bessel van der Kolk MD
Intense relationships also tend to hijack all of a survivor's relating capacity. It is like a state of being burnt out. First, while it is very easy to become attached to a very chaotic and inconsistent person, it is simply not possible to form a consistent internal object representation (feeling memory) about them. When separated from the intense partner, the urge to make contact is usually intense because it is a stable feeling memory (or internal object) that makes separation from an important other person tolerable in any circumstance.
Second. the survivor can come to find that it can be almost impossible to relate to anyone, even family or old friends, except superficially. There is a biological craving for intensity that no normal relationship will satisfy. This provides a feeling of being totally alone, and totally empty. At first, only going back to the primary aggressor can overcome it. It would be normal in this state to believe that something is horribly wrong with leaving (even if it seems equally true that something is horribly wrong with staying. If it can be understood that abstinence from unnatural intensity will eventually restore normal relating capacity, the period of distress can be better endured.