You both are insecure about something. Your insecurities magnify his insecurities and vice versa. That means each time you do something because you are insecure, he sees you doing it. When he sees you doing it, he has the experience of feeling his own insecurities even more intensely than he did before he saw you doing it. He reacts to his experience of feeling the increased intensity of his own insecurities by doing something. You see that "something" he does, and it heightens your own feeling of insecurity.
It's a cycle both of you fuel with your reactions to each other. It takes a lot of energy to keep the cycle going. Both of you get exhausted from it.