Like Kaysen, herself, states as a reason for being at the McLean hospital, reality was becoming "too dense" for her. Susanna Kaysen, the daughter of a powerful man from D.C.
Kaysen experienced an increase in depressive emotions and mood swings that would lead her to basically shift from one reality to another. This condition is later diagnosed as borderline personality disorder. This disorder is often associated with young, intelligent women who crumble apart during a stressful situation. They have already lived through mood swings, depression, pressure, and self-image issues. They also have an inability to establish strong bonds with friends and family, and tend to be suicidal.
This being said, Susanna is definitely someone who could benefit of a place like McLean, whether she likes it or not, because it could help her at least socialize in a contained environment. However, the practice of psychiatry in the time Kaysen was a teenager was not the most researched, nor the best in terms of practices.
Girl, Interrupted narrates Kaysen's experiences in that hospital, to which she arrived by the suggestion of a complete stranger. Overall, the experiences she lives there are typical of any coming of age girl with deeply rooted issues. However, it is clear that the message she intends to bring is that there is never a cure for anyone who has ever experienced a deep depression. It is a lifelong battle and it should be always kept in check.