Biff and Happy love their mother as dutiful sons do but they think she is weak. They never realize that through the years she is covering for her husband's lack of job success and extra marital affair(s). She is their model of what a wife should be and she falls short in their eyes. They see the relationship between the parents and know there are flaws. Assuming Linda is weak and that the marriage has elements of a sham, they cannot form their own relationships based on Linda's cover up and the model of their parents.