An old saying, no doubt heralding from the time of your great-great-great grandfather, that means basically, "Be nice, kid!"
Just as flies are more likely to be attracted to spilled honey than to spilled vinegar, you're more likely to be able to attract favorable attention (whether we're talking about impressing strangers, getting your hunny-bunny to like you, persuading your boss to give you a raise, keeping the cops from busting yer ass, etc.) if you act like a nice, personable human being rather than a mumbly old sourpuss.