Why does Achebe spend so much time setting up the dichotomy between Okonkwo and Nwoye?

Okonkwo finds Nwoye effeminate and weak. He finds his adopted son, Ikemefuna, has the traits of "manliness". I think it bothers Okonkwo to no end that his genetic son is the weak one. Nwoye is a constant reminder to Okonkwo of what he can't control. Okonkwo's eldest son is a reflection of himself which he despises, yet somewhere hidden far below his own soul recognizes.