You're quite correct. The ending -ㄴ 다고 is used when quoting what sb(including yourself) has said or written. (verb + ㄴ 다고, noun + (이)라고) 사랑하다 사랑한다 사랑한다고 사랑한다고 말했다/했다/썼다, etc. However, here it conveys stronger sense than just "사랑해". "널 사랑한다고" without such endings 말했다/했다 is used when sb is full of emotion, and there are chances that sb shouted this phrase while crying(like on soap operas^^/) (sb = somebody) Here is another example, A : 오빠, 사랑해(whispering) = Oppa, I love you. B : 뭐라고? = What did you say? A : 사랑해(whispering again) B : 뭐? = what? A : 널 사랑한다고, idiot! (^^/) = I said "I love you", idiot!! \^o^/