We all, at times, make quick conclusions about someone, only to find out that we were wrong.
When we act on snap judgments, it's not a good feeling, and it tends to stick with us whether we're on the assuming or receiving end.
Our brains have evolved to make quick judgments about the people we meet to sort them into understandable categories. It's how our ancestors decided if something was safe, or if we were in danger. This instinct is entirely valid in those dark alley moments. However, in everyday encounters, we aren't in the prey-predator situation, and yet our brain still has this innate need to categorize.
To go from unconscious judging to conscious observation, when meeting someone for the first time to ask yourself, 'How do I want to think and how do I want to act? What is the other person showing me?' In other words, take five seconds to really observe before jumping to conclusions.