Musical improvisation is a widely used practice within many musical genres and cultures. But although there have been several studies on the experience and psychological benefits of music listening and music making, few have focused specifically on musical improvisation outside the practices of music therapy and jazz improvisation. In this study, we aim to explore the phenomenology of musical improvisation to find the possible psychological motives and everyday mental health functions related to it. Semi-structured interviews were recorded with 11 musicians (2 female/10 male, mean age 27.8 years) and analyzed using Interpretative Phenomenological Analysis. The analysis found two master themes (with five emergent themes): self-actualization (with need for growth, self-expression, and peak experiences) and intersubjectivity (with receptivity and rapport). In this empirical study, we found that musical improvisation provides a medium for self-expression and self-exploration, and by creating an intersubjective field within the improvisers and listeners. These motives also reliably trigger peak and optimal experiences, such as flow state.