"I don't understand what you're getting so pissy about."

Trafalgar Law is not interested in being a bodyguard, babysitter, or anything of the sort. So truthfully, he isn't quite sure what motivated him to accept Luffy's request to keep an eye on you for a while. Is it because you were kind of hot? Is he truly that vain?

Regardless, your ungrateful reaction to the matter is both unexpected and unappreciated, hence the glare you were receiving as he leaned against the wall opposite of you.