I think that Peck is at his best when he gives us good reasons to avoid immediate gratification. Life needs to be lived in accordance with a certain discipline in order to be emotionally fulfilling. This is what Catholic spiritual writers refer to as a plan of life. If we depart from this road, we will encounter many emotional and spiritual problems. The disciplined life entails mortification, admittedly a word the author does not use. We need to learn to say no to ourselves. This is clearly best taught in childhood! We also need to take full responsibility for our actions.
Because we are rational creatures, Peck is calling us to live according to right reason. This should govern all aspects of our lives including the way we love. In contrast with romantic infatuation, real love is something we partake of consciously. Mere infatuation does not last. Authentic love is of the will and when we will to love, that can be demanding. If married couples follow Peck's recommendations, there will be far fewer divorces.
Life, says Peck, is difficult. We need to face the pain that life brings us. Escapism and fantasy ("if only I had...") are not the answer to the problem of pain. Only by accepting our sufferings on the road less travelled do we grow as human beings. To put it another way, the road Peck recommends seems very much like the Way of the Cross.
It all depends on how we suffer. We may suffer in a neurotic way and blame ourselves for all the sufferings we encounter. Here lies the way of anxiety and depression. We may also blame others for all of our problems. This group suffer from what Peck calls character disorder. Perhaps we might call this personality disorder these days. The way to grow as a human being while suffering is not to blame ourselves or others but to take responsibility without being crushed. In Catholic spirituality, we unite our sufferings with those of Our Lord Jesus Christ.
The weakest sections of this book is when Peck deals with spirituality. Then again, Peck was
not an orthodox Christian, as far as I am aware.