It’s like the two person psychology and the idea of co creation they are based on a philosophy of no individual personal power. One isn’t who they are as an individual, instead one just becomes part of relationship and is powerless to be the core of who they are.
It is said that the 3 year old is assentive whilst the 4 year old is assertive. People with assertion problems maybe fixated at this stage.
If she believes this (with or without his encouragement) she finds herself in the same relationship dynamics as the teenager who is not economically separate from the parents. The marriage will then suffer.