I can't speak for the girlfriend. I imagine there are lots of different 'reasons'.
I did it for years. I confused lots of things for love. I was afraid and insecure that my world would fall apart and that I could never have what I wanted, so I tried to control everything to make sure I got it. It didn't work btw. I didn't get what I wanted, my world did fall apart - and low and behold I lived to tell about it
I think boundaries are the single biggest issue I struggle with. Here is an article that really resonates with me. It speaks about unhealthy emotional boundaries and how that effects relationships.
Emotional Boundaries
All of it hits home with me but this section talks about the controlling.
Over-Responsibility and Guilt
One characteristic of growing up in a dysfunctional household is that we may learn to feel guilty if we fail to ensure the success and happiness of other members of the household. We may feel responsible or be made to feel responsible for the failure or unhappiness of others. Thus, in adulthood, we may come to feel or be made to feel responsible for our partner's failures. The guilt we feel when our partner fails may drive us to keep tearing down our personal boundaries so that we are always available to the other person. When we feel the pain, the guilt, the anger of being overly responsible for another person's behaviour or life experiences, we may seek alleviate this feeling by rescuing them from the consequences of their behaviour as we learned in our family of origin. Thereby depriving them of one of the most important features of an independent, healthy and mature life, the ability to make our own life choices, accepting the responsibility for and the consequences of our/their decisions. Or we may bear the burden of their unacceptable behaviour for many years.
I was super responsible and doing things and taking care of things was easy for me. I met and married a man that wasn't so good at it and didn't care if I did it. It 'seemed' to work at first. I felt warm and fuzzy doing it, he felt taken care of and loved. I took over all kinds of things. More and more things. Alcoholism progressed, children arrived. I took over more and more. He did less and less. It was a disaster of my own making. It hurt both him and I very badly.