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.
It would certainly have helped if the show had started with Luffy's backstory