 While not mentioned in the film, a few books released in the 70s at  the time of the films release mentioned that Cheng Chao An was  forced to make his promise to his mother after his father was killed  in a fight. In order to continue the family name, Chengs mother  wanted to make sure he would not fall to the same fate and that hed  live to raise his own family. Film historian Bey Logan even mentions  this in an audio commentary.