It is in character for Luffy to say that again and again, he is not doing it for the gags, but to remind himself of what his goal is. The first step to fullfill your dream is to internalize it.
That is why during supernova vs Marine in Saboady, Kidd respects Luffy because Luffy had guts to speak about finding one piece so casually as if its a daily ritual. Its also the same way Luffy repeats that he is going to kick *insert current opponent's* ass.
If you look at the story, most of the major decision in the story Luffy has took, he have said everything aloud and clear.