When it comes to how we approach life, how we view the world -- some folks say there are only two categories.
By this way of thinking, you are either an optimist (seeing your milkshake decanter as being half-full) or you are a pessimist (already mourning the milkshake you can see, through your salty, bitter tears, as being half-depleted).
It’s easy -- and tempting as hell - to label people’s perceived worldviews. If we can quickly assign people into one category or another, then we can expect them to behave in one of a certain number of limited ways.
The problem is -- this almost never happens. People don’t often behave the way our labeling would dictate that they ought to. Labeling someone as optimistic or pessimistic doesn’t allow for the inherent weirdo-ness that comes with being human. It sets a person up to surprise you, like in this study where people labeled as pessimists were viewed as having more successful marriages.