Why did Kelly maintain that all humans are like scientists?
George Kelly was an American psychologist, and is considered as the father of cognitive clinical psychology. He gave the theory of personality called Personal Construct Psychology.
Personal construct theory states the personality and cognition of a person. Kelly notched a psychotherapy approach to the theory and devised a technique called the repertory grid interview. This technique aided patients to analyze their own relation with the world, with minimal intervention.
Kelly maintained that each individual's capacity to understand his own personal psychology is up to his own level of perception from his own experience. The individual enacts like a scientist and try to test the accuracy of his own constructed knowledge, by analyzing his own performed actions.
He tries to analyze if the interpretations is based on his own predictions or not. This method of discovering and correcting constructs is quite scientific, and hence, it was maintained by Kelly that every human is like a scientist in his own way.