You should never be so trusting with every person that you meet. There are many good people in this world, however, there are many people that do not have anyone's best interests in mind.
Discretion: The freedom to decide what should be done in a particular situation.
Discernment: The ability to judge well. In Christian contexts, perception in the absence of judgment with a view to obtaining spiritual direction and understanding.
Do you not let your wanting to believe the best in people override your common sense. Listen to what people say - watch what they do - see how they treat others.
