When I first discovered the periodic table in high school, I was fascinated.
Here was a system that could bring order to chaos, which could make sense out of a mass of data. All you had to do is to organize the information, and put it into categories - into little boxes if you will - and almost magically it enabled you to predict the behaviour of new elements just on the basis of which group they belonged to.
As I went on to engineering school, and taught myself programming, this logic stuck. The best way to deal with information is to categorize it. The lure of the if-then-else logic was amazing. If it-looks-like-this then it-must-be-one-of-those. QED!
It is indeed a powerful construct and modern science would not be possible without it.
The only problem is that it does not apply so neatly to people - but driven by its apparent universal success, we try to apply it anyway, and that causes problems.
Have you noticed that as soon as you meet a new person, you almost immediately and unconsciously start trying to pigeonhole him or her into a category. Is he from this school ? which city did she grow up ? is he local or did he move-in ? Until we have fitted him into one of these boxes, we are not sure how to deal with him.
People, of course, are more complex. They can belong to several "boxes" at the same time, and some will defy categorization completely. There is no shortcut to dealing with them if we are really sincere. We cannot put familiar tags on them and be done with it. Knowing people takes time, and energy, and patience - and most times we are just too lazy to make the effort.