Making the decision to pursue your academic field or area of professional interest probably wasn’t an instantaneous decision (or a matter resolved by flipping a coin). So think about your personal investment into your major/area of work and your sentiments regarding what you have chosen to study or actively practice that in the professional world, and the rationale for your decision may come more easily to you. Reply with reasons that are both earnest and honest, while showing enthusiasm. Additionally, even though it might be tempting to explain your choice using a lengthy story, try to keep your reasons reasonably brief and concise.