The sign of Cancer loves to give everyone the benefit of the doubt. Chances are, someone or something has hurt you because of this trait in the past.
You're just trying to be fair, and avoid pre-judging anyone based on appearance or assumptions. But it can come back to bite you.
This can spill over into your love life as well. Rather than assuming the best (or worst) of anyone right off the bat, it's best to allow partners to prove their trustworthiness before diving in. You'll be happier, and won't get hurt as badly.