Assuming the worst about people is one of the most counter productive things we do to undercut progress. From my experience, the assumptions are often wrong, and once we realize it, embarrassing.
It’s often done when we lack understanding of the other person. Simply put, the more we know, the more we understand. Be it another political party, the person who is “just out to get me,” or your neighbor, the more we understand of their perspective, motives and beliefs, the more patient we are with them and more positive we think about them.
As a result, solutions are reached more quickly for everyone involved.
Once I realized this, my perspective of people overall became more positive, which is saying something since I’m a pretty optimistic guy, and my ability to lead improved dramatically. And most importantly, my wife and I have successfully avoided arguments rooted in misunderstanding of one another. The benefits are truly endless I believe.
So, my challenge to you is to watch out for this in yourself and others. If you’re often guilty of this, make it a point to work on it for one week. I guarantee your stress will reduce, your attitude will improve and you will accomplish more.
Joel Fortner's blog 
This is such a great point Joel. It makes me think of the Jim Rohn quote that says “We don’t see things as they really are, we see things as we really are.” When we are believing the worst of others its often a reflection of our self image.
Remarkably, when we always look for the best in others, not only do we usually find it, but our own self-image improves naturally.
I’ve experienced that!