The Carl Jung theory and study of psychology. If you ever get a chance to study the facets of psychology, it is interesting to delve into the conceptualisation of Carl Jung's balance of personality.
Jung also introduced the concept of personality types, and began to address personality development throughout the lifespan.
If you take to trying to further understand yourself, like Myers Briggs personality tests, you would further like the multi dimensions of what Carl Jung presents through his years of experience and conceptual works.