Didn't you read chapter 18 ? or was it 19 ? Anyway, it was when he spoke with chooper about the whole ' monster ' thing... We got a little flashback where some people were hunting Luffy trying to kill him when he was saved by Shanks.So it's safe to say that he went through some sh°° that made him what he is today.He started cold, but toned it a bit down when he meet chooper.