Luffy was awaken rather painfully from his grandfather - Vice-Admiral Monkey D. Garp. Sanji questioned Luffy on why would it hurt since he was a rubber man, punches shouldn't hurt. However, Garp calls it a fist of love, so it would and should hurt.
Luffy is pretty scared of his grandfather, so he tells the other Mugiwara to don't attack Garp. Then he gets into a long standing argument with the vice-admiral about his dream of becoming a pirate, when it was apparently Garp-s intention after tossing him into countless ravines, jungles and into the sky with balloons to train him to become a proficient marine. Garp blames it on the influence of Shanks. So they were arguing and suddenly they fell asleep. Possibly a characteristics of the family, since it is similar to how Ace falls asleep into his food. After beating Luffy again for his rudeness towards him, Garp reveals a bit more information on the level of Pirate that Shanks is at.