The Benefit of the Doubt.

-Undermines confidence. 
-Assumes untruths.
-Builds walls in the relationship.
-Breaks trust.

When you add benefit to doubt you:
-Seek to understand
-Root for the other person
-instill confidence in the other person
-gain respect from others.

The challenge with automatically writing somebody off, assuming first they have flaws and can't be completely trusted is that you will almost certainly be right.

Then again, you will be right about yourself too.