This study is conducted to explore what factors play key role in developing expertise and how individuals develop their own expertise through work experience. 18 life insurance sales experts are interviewed and themes on the process of expertise development and key factors are analyzed. Research findings are as follows. First, expertise is developed through three different interactive phases between the self value and the occupation value. When individuals were first introduced to the job, their self and the job interact through which the insurance sales experts went through unification phase as they performed their tasks. Second, insurance sales experts showed extremely strong goal-pursuing behavior, acting as a driving force behind development of expert through goal setting and goal accomplishment. Third, the continuous formation of positive attitudes and desirable habits is critical in maintaining and improving the level of expertise.