We put people into boxes because it is convenient. It's easier for our mental model of the world to say to yourself things like “This guy works with computers, maybe he can tell me how to fix my printer.” or “This person is from [school] and they are very arrogant.” or “You're from [country]? You guys do [that country's thing] right?” or “This person is a supporter of [politician] so he must support all the things that politician does, even the things I hate.” Or in the modern world, there are even worse stereotypes.
Boxes and stereotypes are convenient because it means we don't need to relearn our reactions and opinions when presented with a new person. We simply choose a convenient box to put them in and then we convince ourselves we are treating them appropriately.
But the real world is not so discrete. The real world is a lot more granular, a lot more nuanced. While it is inconvenient for your mental model, people will usually not fit exactly into any of your preconceived boxes. Or they may fit in multiple boxes. Or maybe none at all.
Every time you put a person into a box, you discredit her by some amount. In your mind, you shave off some of who he is a person so that he better fits your box. And for many instances, maybe that's okay. Maybe you don't have any significant interaction with that person, and the simpler mental model is sufficient. You don't need to understand the character nuances of every grocery store cashier for example.
But beyond that, in order to pursue any kind of meaningful interaction with another person, that is insufficient. We must be willing to set aside whatever box you have put a person into and be willing to understand all those little bits we set aside. Those nuances characterize and define the individual, not whatever box we have put him into.