"Why meditation can be harmful to certain types of personality" is a sensitive subject. In the capacity of a student of psychology and a reader of Carl Jung, I venture to tackle this subject. Certainly this is an exploration rather than any attempt to draw any conclusive recommendation.
Carl Jung had warned against Westerners mindlessly follow the Eastern path of enlightenment which boiled down to deep meditation. Two types of personality had been warned against by Jung: firstly those who have a particularly strong ego and secondly those have very weak ego. In short, according to Jung, it would be harmful for personalities at the extreme. For those who have super-strong ego, deep meditation can boost up his ego even further, and he can become more ruthless in his daily life as a result of his meditation. Meditation on love and compassion become love and compassion towards oneself. For those who have super-weak ego (like someone in depression), any unwelcoming images released from the unconscious can scare his ego to deep depression; in addition he may become psychologically dependent on his teacher, coach or master, however it is named. When our personality is too lop-sided, according to Jung, a psychological breakdown becomes a real possibility.
For those with a more balanced ego (which means most of us), meditation can change our personality in a positive way. We become knowing ourselves better. We can appreciate our own kindness and at the same time accept the fact that we have our darker side. We will become more compassionate to the sufferings of others and at the same time won't overate our success. All at our free choice. A self-actualized person choosing to become a better person despite accepting his own shortcomings. In short, we will become a better person without the need to seek other's approval. In Jung's terminology: self actualization.
In future posts, I shall discuss how we can achieve these positive results with the help of appropriate meditation techniques.