Simple example: say we want to analyze the pattern of social stratification in a society. The psychologist typically explains stratification by pointing to greed or selfishness, but that's not adequate from a sociological perspective. Regardless of the psychological tendencies of people in a society, if certain social conditions and relations do not exist, then those psychological tendencies cannot contribute to social stratification. --No matter how greedy or selfish a person is in a hunting and gathering society, the social structure of that society makes it impossible to produce the rigid and gross inequalities we see in our society. From the sociological perspective, what we need to analyze are the social structures that make possible certain social patterns and discourage others; the consideration of psychological tendencies is completely irrelevant to that analysis.
Now, this is not to say that psychology is trivial; if we want to understand individual thought and behavior, psychology is crucial. But that psychological analysis cannot replace sociological analysis of the way society "works".
Miles