Consider for a moment that person you tend to assume the worst when you think about their motivation for doing/not doing something. Now, consider your best friend and consider how you would react if they did the exact same thing as the first person. Would you assumption be different?
We like to believe that that our feelings and actions are based in facts and knowledge when in fact they are much more based on prior experiences (not necessarily facts) and interpretations we made about those experiences. What I find interesting is that we tend to either assume the best or the worst and find it difficult to remain neutral. We tend to perceive that people are for us or against us. Once someone crosses the divde from “for” to “against” we tend to go back and reinterpret our history with them to read their behavior toward us in an completely new light. Some times this is warranted. Other times it is not.
Can we live without making assumptions? No. But, our challenge is being humble about those assumptions and willing to be flexible (assuming the best) as much as possible as 1 Corinthians 13 calls us to. Such a move should not be naive but merely recognizing that we ought to be equally suspicious about our own assumptions.
Musings on Faith and Psychology 
Whatever the degree of accuracy our predictive abilities may have to serve us well and keep us out of danger, they are also stained with our inherited legacy of longing for omniscience. Only humility permits us to hold our mental “mileposts” less firmly if they’re challenged.