I have never met a man so ignorant that I couldn't learn something from him.
When I judge another I close off lines of communication with that person. I reduce our ability to learn from each other and thus to grow. The more I judge the more I turn my sight inward, reducing what I can perceive of the world around me.
The more I learn from another the closer I come to having some understanding of that person. I can never fully understand another, as I have not experienced what they have. That difference in experience gives each of us a different perspective. By comparing our perspectives we can move closer to understanding.