Its hard to escape our own orbit. Its hard to see the world objectively apart from our wishes, desires, and personalities. We tend to assume that others think and feel similarly as we ourselves do. This is not a new idea--the concept of projection is an old one.

Projection can be thought of in other ways too, not necessarily psychodynamic. It may be, too, that  as observers, we have only so many constructs for interpreting  the world. We use the same ideas to interpret others as we do to understand ourselves.   Ask a person to describe another person, and the description will tell more about the describer than the describee.

One impediment in understanding the inner life of others, is that  we have different brains than others do--our organ of experience is different. Our hardware is never quite the same as the next person Our brain may have a different arousal level, different proneness to emotionality, different propensities to experience certain types of emotion, and different ability to process different types of information. So it is difficult to get into their shoes, mentally.

But we can improve in our understanding.  Like anything else, it is a matter of practice, listening intently without judging, exposing yourself to  many different situations, and reading books by people who themselves learned how to do it.

It takes great discipline for a therapist to "decenter", and attempt to register different configurations of personality. Familiarizing oneself with psychological taxonomists helps--- It helps to read Kraepelin, Sheldon, David Shapiro, Millon (particularly) other personality theorists as well as some literature. Chaucer is said to be a good, or really one of the first, taxonomers in literature, and Shakespeare's characters seem differentiated, with a full scope.  But for the most part, authors suffer the same problem.  Their characters tend to be extensions of the authors themselves.

Actually, computers are probably best at sorting human types. Along this lines, a famous (and for some very upsetting) paper by Meehl indicating the superiority of statistical over expert diagnosis, is germane.

The ability to recognize human types gets better over time. When we are children, we are particularly bad at interpretation, and getting beyond our own perspectives. This is the Piagetian notion of egocentricity. Adolescents are a little better by virtue of their increasing maturity and experience. However, the self-encapsulated thinking style of adolescents, often a source of despair or humor to parents or teachers, tends to be self-referential, and therefore not flexible.

The more complete or enclosed or recognizably self sufficient a person's system of thought or beliefs,  the more they are to be dense to the inner world of others. The more religiously or politically or morally doctrinaire an individual, the worse they tend to be. The best attitude is one of non-judgmental observation.

Another interesting phenomena is that we tend to absorb the perceptual style of those around us.We tend to pick up others way of  seeing the social world. Say, if you spend time with someone who sees the world through money, and you begin to see people in terms of economic ideas. If you spend time around a person who is paranoid, you begin to see the world as full of deceit. These thoughts are similar to that of Freuds notion of introjection. We sort of absorb those people we are bonded with.

Even great writers seem to recreate worlds of multiple selves. They have as hard time escaping themselves as any others.  The characters in most any authors books seem to be varied extensions of  their authors.  A character in an Aldous Huxley book is different from a character in a Philip Roth book, and all the characters of each author will have a certain similarity with other characters invented by that author. The character in a Huxley novel will be airily interested in ideas, whereas that of , say, a Christopher Isherwood book is likely to be a humorous, bright, self-aware, slightly amoral individual with an actively observed inner life. Meanwhile, the world of  Hemingway seems peopled with characters with alcohol problems, a proneness to fight, and a love of fishing.

In striving to be more aware of the variety of individuals in this world--and that is not a goal everyone will find important--but, in striving, if that is important, to be discerning of individual types, the best we can do is develop habits of mind that let us passively register other people without too quickly jumptng to conclusions about them. I'd say that perhaps the best single person to read, for improving this ability, is Theodore Millon, who, unfortunately just died. He was instrumental in developing the Axis II portion of the DSM-III. A lithuanian, I believe.