Reason number 1: You've stripped the relationship context. The point of dialogue is to characterize and drive the plot. A big difference between Japanese and English is that Japan has more telling pronouns and honorifics that suggest the relationship between characters and a little about their personality. In English you just say "I" or maybe "me", which is doing nothing but communicating the subject of the sentience.
So to compensate for this in English, you'll have to elaborate in other areas of communication. Instead of a straight translation of "hai!" to "yes" you can suggest the mentality and character dynamics through their responses. "Okie-dokie" "Agreed" "Yes, Sir!"