User blog comment:Samium/Nico Robin, human in a world of characters (Essay)/@comment-5239749-20170106210337/@comment-27923483-20170106213833

Again, look to Chapter 398 where Luffy says " I know who Robin's enemy is."

Yes that is the easiest arguement to make,  but do you believe it?

Often One Piece merely tells us that Luffy understands, rather than actually SHOWING us. Needless to say this is one of the most important qualities of good story-telling ... SHOW DON'T TELL.

We don't know until episode 490 (chapter whatever) that Luffy would understand principles like dispair, or sorrow, or oppression, and the hardships of his past never really seem to become part of his character, the same way that they do with Sabo or Ace.

Simply put,  he lives in his happy place waaaay to much of the time. So when he is all of a sudden sincere, it feels contrived. For a while, I think the show was able to get away with it, but it's gotten to be a pattern by now.