Originally, Regulus only needed to be the little master of the Black family. His older brother, Sirius, had always been the intended heir of their father and mother, and Regulus had looked up to him as a role model.
Even though his brother became increasingly rebellious and argued with their mother more and more, their mother always believed that Sirius was simply going through a temporary phase of bewilderment—until Sirius was sorted into Gryffindor.
For a pure-blood Slytherin family to produce a Gryffindor was a disgrace. Thus, the position of heir, along with their mother's expectations, fell upon Regulus.
Chapter 50