The word "nice" has an unusual history in the English language. Originally a term for "foolish", its meaning over the centuries has morphed from "wanton" to "reserved" to "fastidious". These days, it has become a somewhat bland and opaque description of personality: "she's really nice."
At the topmost level, there are two types of people in the world: Those who think personality types can be categorized and those who can't. Among those in the first group are psychologists who began developing a system for classifying personality traits based on an analysis of language way back in the 1880s.