The education model in the 21st century shall be learner-centered. Learners are expected to be independent to engage in self-directed learning with the integration of technological tools in developing necessary 21st century skills. However, the foundation of this education model shall not be neglected as positive emotion and motivation are the determinants of achieving the desired outcomes. The researcher applied a comparative review method to contrast the three learning theories, namely (1) humanistic theory (2) social constructivism theory, and (3) self-determination theory which are significant in the self-directed learning context. The inference of these theories on personal and social development, emotional stability, as well as the motivation of learners in the 21st century was also examined in this paper. The outcomes of this review paper benefit educators in the curriculum and pedagogy design, as well as aid researchers in understanding phenomena and formulating appropriate research frameworks for future studies. For understanding emotional stability and motivation among the 21st century learners, the researcher concluded that the humanistic aspect of learning shall not be compromised. Furthermore, the cause-and-effect of the emotion and motivation constructs deserves a more detailed empirical investigation.