They could also be stressing those words for the opposite reason, like when they stressed Kate saying "I'm not good with babies" and then she has a baby in the flash-forward the very same episode. Maybe stressing Jin saying "I will never leave Sun" (or whatever the exact phrasing was) is actually alluding to some sort of betrayal where Jin does abandon Sun for her own safety. They could have been wanting to show us later just how hard this would be for Jin to do...