Why is it important that the study of behaviour be systematic?

People in general are both consciously and unconsciously students of behavior. We all observe and make judgements about what we see others do. From that, we attempt to interpret their actions and predict what they might do under different circumstances. Because we spend so much time doing this, we tend to make generalizations about other people and their behavior based on our observations. This can be something which negatively affects our ability to understand why people do what they do. That is to say, it influences the development of certain beliefs that fail to help us explain the behavior of others.
A systematic approach to the study of behavior would help us to eliminate the tendency to base our understanding of behavior off of our own intuition. It assumes that there is an underlying pattern to human behavior and that it is not random. This approach to studying behavior will improve our knowledge of facts and form a base from which to make more accurate predictions about behavior. People are not all alike; they have different beliefs and they act according to what they believe to be in their best interest. Through systematic study, we can look at relationships, cause and effect, and scientific evidence that we can then base our conclusions on. 
In a systematic study, data is gathered under controlled conditions, which allow us to control for variables, of which there are many when we are talking about individual people. When we attempt to use our intuition or "gut" to predict the actions or behavior of individual people, we fail to take into account the underlying consistencies that make it easier to accurately predict an outcome. By using a systematic approach, we can interpret data in a more rigorous manner.