The figure/ground illusion shows how we literally cannot see two different things in the same set of stimuli. This feature of psychology generalizes to how we see people.
...One of the interesting things about human social psychology is that, in many regards, we tend to over-simplify stimuli in our social worlds - seeing things that could be conceptualized as complex and nuanced as simple and categorical. For instance, in many ways, we divide people into the category of “on my team” or “not” per the powerful ingroup/outgroup phenomenon (Billig & Tajfel, 1973). Quickly and automatically, people divide folks into these categories - and research has shown that we treat people very differently if they are in our (psychologically constructed) group or not.