Someone knocks on your door, bleeding from a wound on their arm. Are the a person in need or an armed assailant waiting to gain entry into your home?

A person flagging down help on the side of the road. Are they going to do something while you're in a vulnerable position changing a tire?

Most any interactions where "help" is more than verbal information transfer usually lead to situations where the helping party is put into a position of vulnerability. To help someone is to literally "go out on a limb" i.e. put yourself in a precarious position.