Who would have thought that referring to your parents as your "best friends" would be a bad thing? Turns out that it is as it signals that the person's psychological development has stopped. The harm comes in that the person is unable to honor their own reactions when the parent's imperfections, insecurities or weakness show up. To not react the person must disconnect internally from their annoyance and find a way to excuse the behavior. Rather than give themselves the right to give voice to their reaction, they become overly concerned with protecting the parent from distress, and find a way to excuse bad behavior. In short, their guilt trumps their anger.
The normal development path follows a different course. Children idealize their parents because they are dependent on them for survival. They see themselves through the eyes and reactions of their parents. They feel good when they please mom and dad and feel distress when mom and dad disapprove of what they do. They learn to hide their own natural reactions and trust mom and dad's reactions more than their own. They are too young and vulnerable to trust themselves, especially without mom and dad encouraging and validating the truth in the child's reactions. The child learns to please to survive and protects the parents as a means to protect themselves.
This connection based on idealization over time gives rise to the process of learning that mom and dad are each imperfect people, only some good and some bad. Each is capable of giving love and equally capable of doing harm. During the adolescent rebellion, as a result of comparing one's parents rules and actions to their friend's parents behavior, the child starts to listen to their own reactions and challenge mom and dad. If the parent accepts the challenges, and validates the child's reactions, then the child starts to develop an independent sense of self. They feel strong in their beliefs in what they see. They learn to listen to themselves without discounting others and learn the art of negotiation and compromise.
Unfortunately, it doesn't often end this way. Many parent's succumb to their own insecurities, and take too personally the bad style of the adolescent. Being yelled at or ignored is not easy, and not a self-respecting style. A secure parent helps the teen to separate their style from their content. A statement ,"if you curse at me again, you lose the car and cell phone for a week" is followed by "Now tell me why you are so angry so we can work it out". An insecure parent will yell back, retreat, or severely punish the teen without caring about the problem that got the bad exchanges started in the first place. They will focus on the demand for respect rather than solve the problem. In this case, the teen feels ignored and they retreat, only to act out even stronger when the next problem emerges. This pattern leads to isolation and distance in relationships, the exact opposite outcome that neither teen or parent wants.