In "Kafka on the Shore," Haruki Murakami explores the profound and often mysterious feelings that arise within us. The quote conveys a sense of an unfamiliar and powerful emotion that begins to grow in the speaker's heart. This force is described as a "strange, terrific" presence, suggesting that it is both exhilarating and potentially overwhelming, taking on a life of its own.

The imagery of the heart expanding and contracting independently highlights the theme of surrendering to emotions that cannot be controlled. It encapsulates the essence of human experience, where feelings can emerge unexpectedly, profoundly affecting one's inner world. This reflects Murakami's fascination with the complexities of the human psyche and the enigmatic nature of our emotions.