Reference: R: Psychological Types by Carl Jung
Carl Jung, based on his clinical practice found two characterstics of human beings which determine their relationship between themselves (subject) and the outside world (object). The characteristics are called introversion and extraversion.
Each human being always leans closer towards one of them, never being 100% introverted or extraverted.
The extraverted attitude has a positive relation to the object – it is drawn to it. An extroverted person relates his thoughts to the object and can only reach an idea through the object. The more he expands himself, the more objects he can relate to, and the more energy he gains.
The introverted attitude has a negative relation to the object – it withdraws from it. An introvert generates thoughts from within, without the object which only drains him. The more he withdraws form the outside objects, them more energy he conserves.
Relevant notes:
What may cause these differences may be differences in sensitivity to stimuli