It is ironic that Tris, an Abnegation born woman, would have an aptitude for Dauntless let alone pick Dauntless for her faction. Abnegation and Dauntless are polar opposites, Abnegation being all about care, safety, and kindness and Dauntless being aggressive, frightful, and strong. There is rarely anyone from Abnegation who would choose Dauntless at the choosing ceremony or even have an aptitude for Dauntless but Tris is special, she is Divergent. This Irony is what makes this book work and build a great story line.