re: why that one sentence ended in a period, rhetorical questions can end in a question mark or a period depending the intonation you want it read with. in the case of "우리가 왜 남이야?" vs "우리가 왜 남이야.", ending with a period reads with the confidence that he's never considered them strangers. imo the sentence is better translated as "why would we be strangers" with the period vs question mark being up to your discretion.