Toxic relationships are addictive because drama is addictive. Like narcotics or gambling, drama is unpredictable; it is numbing and distracting, and it hits you with unexpected rewards of joy or excitement.
What's worse, is that we become desensitized to drama. We need to find greater and greater conflicts to prove to ourselves that we're loved. The old conflicts will no longer suffice. You started out with a fight about who takes out the garbage. Now he takes out the garbage. But you still feel insecure and unloved. So you start a fight over how often he calls his mother. So he stops calling his mother (around you at least). But that insecurity remains. So you must up the ante again. Time to piss in his favorite pair of shoes and see how he takes that.
Eventually, the drama reaches a boiling point and the relationship will begin to painfully evaporate, scalding everyone involved.
But something else happens when we're caught up in a drama spiral. As we up the ante and the drama increases, we become more emotionally dependent on the person, not less. We invest so much into the drama that we come to believe that our partner is far more important to our well being than they actually are.
Drama is therefore a psychological prism—a funhouse mirror—skewing the meaning that a relationship brings us. In our eyes, this person or this group or this activity is everything we need, when in reality, it's probably the one relationship that likely harms us the most.
Incidentally, people who don't know how to let go of a relationship are often those who were in a relationship with someone who was either abusive or completely disinterested. That's because, in these relationships, a breakup changes nothing. When they were together, the person spent all of their time and energy trying to win their partner over. After they split, they continue spending all of their time and energy trying to win their partner over. Same shit, different day.
Similarly, people who are unable to accept the loss of their relationship will badger their ex and instigate drama with them to re-live the sensation of that relationship. But they need to create that drama again and again to keep that feeling alive.
Drama, of course, can infect other relationships as well. People create drama at work to overcome their insecurity of not being valuable or appreciated. People create drama with authorities or governments when they feel an existential insecurity. And people create drama with themselves when they imagine they aren't living up to some sort of past glory.