We all make these decisions based on our conception of a person from meeting him - what he said, how he acted, how he made us feel.
Many factors create the atmosphere surrounding a meeting - business or personal.
Most people can only describe the outcome of that atmosphere, but not exactly what caused it. The answer is a mixture of both. However, we can learn to understand what kind of an atmosphere we build around us naturally, and what we can do to influence that impression and change it. Simple - ask a good friend what he thought of you the first time you met. Understanding our starting point for making a good impression is crucial for helping us focus on our weak points - those we can and need to change.