We have evolved to act on first impressions: without them we might have had no second impressions.
It has been estimated that we make thousands of unconscious judgments within the first moments of meeting. Many are wrong.
It is well established that, having formed a first impression, we tend to confirm it by attending to supporting evidence and by ignoring contradictory evidence.
A man who tells you that he always goes by first impressions, is telling you that he is a bad judge of character.
It may be unfair to judge someone’s future behaviour by their past behaviour but it is the best, and often the only, evidence available. You will be right far more often than wrong.