폐쇄적 means "closed; exclusive; cliquish, cliquey; unsociable " The antonym is 개방적(open; frank) 그는 성격이 폐쇄적이다 He is not at all open He is unsociable. 성격이 폐쇄적이라 due to the unsociable personality

said of one' character of being aloof and closed-off, I suppose

성격이 폐쇄적이다. means a person is introspective. Therefore, 성격이 폐쇄적이라 means that "Because someone is introspective person" But 폐쇄적이다 is not good expression. It has negative meaning. 성격이 내성적이다. is better expression.