The emergences of the aspect suffixes le (perfective) and zhe (progressive) were epoch-marking events in the history of Chinese grammar, and their development processes should be seriously studied. This chapter explains the reasons why they emerged at the time they did. Two closely related questions need to be answered: what factors in the earlier grammatical system blocked aspect suffixes from grammaticalizing? What changes in the grammar at the time made it possible for them to come into existence? The motivation and mechanism for their development involves two factors: (a) the overall change of the grammatical system at the time and (b) the concrete grammaticalization process of each aspect suffix.