It is easy to draw assumptions and even easier to draw bad assumptions based on either a lack of facts or selective facts. It is human nature to make assumptions quickly about people or situations. Mature individuals, however are circumspect, think grey and get the relevant facts before drawing any conclusions.
Think about this: How often have you heard something and drawn negative conclusions about an individual or organization without knowing the full story? Why is it that we are far more likely to draw a negative rather than a positive conclusion? And how often have you been wrong when you heard "the rest of the story?"
Good counselors know this and when one party comes in to complain about the other they resist the temptation to believe everything they hear as the full story. It is rarely the full story and without all the facts, impossible to know.
I like to practice the art of thinking grey, keeping an open mind, when I am given one sided facts. If it matters to me I will make the necessary inquiry to ascertain their veracity which is usually a personal conversation with someone. If it does not matter or pertain to me I simply recognize that I don't know the full details and don't desire or need to know them.
Correct assumptions come from knowing the facts first. Incorrect assumptions come from getting facts and assumptions in the wrong order. Resist the temptation as incomplete or wrong facts will lead to the wrong conclusion.