Carl G. Jung believed that there is an indisputable connection between our minds and our bodies. Most people today would agree with that assessment. Our bodies affect our minds, and our minds affect our bodies. Our external situation affects our bodies, which affects our minds, and vice versa.
Taking it a step further, he believed that we affect our physical environment, and that ultimately, we affect the solar system. The solar system affects us, we affect the solar system. Our lives are not "caused" by the stars - the stars do not initiate anything, but there is an ongoing two-way relationship between ourselves and the stars.
Taking it another step further, he believed that the singleness of the solar system paralleled the singleness of human experience, and that "we are one." That there is a single external world, and a single internal world. The single internal world is not accessible by our conscious selves, rather it exists in the unconscious mind. He believed that each person's unconscious mind contains not only personal unconscious, but at a deeper level contains collective material that in inherited - the "collective unconscious." Jung felt that we can learn a lot about the human psyche through astrology, not only because the stars and planets are an external reflection of our inner selves, but also because the inherently symbolic nature of astrology allows us to consciously correlate symbols with unconscious knowledge. Jung felt that astrology is a language of symbols that mediates between levels of consciousness.
You may be familiar with Jung's concept of "archetypes," which make suggestions to the conscious mind as mystical images or symbols, and which spring from "known" patterns of behavior within the collective unconscious. For example, Jung thought that we essentially "know" how to act in a given situation, but that this knowledge is not present in the conscious mind until the situation arises, and it will never be brought to consciousness if the situation never arises. An "instinctual archetype" exists in the unconscious mind that contains the knowledge of how to act in that situation, until the archetype is constellated into the conscious mind. This is different from an "archetypal archetype," which is not instinctual or behavioral, but represents a more complex knowledge or understanding of something that we haven't realized with our conscious mind.
Since much of Jung's work was focused around the collective unconscious and its archetypes, he was probably interested in anything that had a long-standing presence in human history. Since the collective unconscious has been bringing forth astrology into human experience for so many centuries, Jung apparently that it must be important enough to deserve some attention.
Jung associated different archetypes with specific planets, and believed birth charts would generate archetypal images to him that told him something about the subject of the chart. He frequently looked at the birth charts of his patients, and believed that the symbols in the charts made suggestions to him from the collective unconscious about that person's psyche.
Jung believed strongly in the I Ching, and invented the term synchronicity to describe psychic "coincidence."
Carl G. Jung is widely recognized as a genius in the field of psychology who made great strides towards understanding the complex human psyche. His most well-known work formed the basis of Psychological Type, later turned into the popular Myers-Briggs Type Indicator psychometric instrument.