An age-old question which most people have pondered at some point in their lives is "why are we the way we are"? Unfortunately, I can't clue you in on the answer to that question in a really authoritative way, but I can tell you about work that has been done to determine how people's Personality Types are developed.
The more recent studies done around Temperament suggest that we are each born with our temperaments intact. Efforts are being made to prove this.
W. Harold Grant did a lot of work with Jung's theories, and concluded that Jung believed that Personality Type has a developmental process which can be observed through an individual's life. The early phases of our lives help determine the dominance ordering of the four functions (Sensing, Intuition, Thinking and Feeling), and the development of our dominant and auxiliary functions. The later phases help us develop our tertiary and inferior functions.
Let's take a look at Grant's phases of development, using the INFJ Personality Type as an example:
From age 0 - 6 years
At this early age, we use all four of the functions in an indiscriminate fashion. We "try on" the different functions for size, determining which ones work best for us. The little INFJ has not yet emerged as any particular personality type, although his parents may notice trends in behavior which appear to have the characteristics of one or more types.
From 6 - 12 years
During this phase, our dominant function begins to develop and assert itself. Our young INFJ begins to appear dreamy and introspective - he begins to prefer to use his iNtuition to take in information, and he chooses to do this alone (Introverted). The dominant function of "Introverted iNtuition" begins to show itself as the prevailing aspect of his personality.
From 12 - 20 years
The auxiliary function asserts itself as a powerful support to the dominant function. Since all recent studies point towards the importance of a well-developed team of dominant AND auxiliary functions, this is an important time of "self-identification". Research suggests that people without a strong auxiliary function to complement their dominant function have real problems.
In our INFJ example, we see the auxiliary Feeling function come to the front during this phase as a support to the dominant iNtuitive function. Since the INFJ's dominant function is an Information Gathering function, the auxilary function must be a Decision Making one. Without a Decision Making process, we would flounder about and never get anything done! As the auxilary Feeling process comes forth, the INFJ begins to develop the ability to make decisions based on his personal value system. This auxiliary decision making process will be Extraverted, since the dominant function is Introverted. Since the decision making function is Extraverted, our subject now emerges as a "Judger", rather than a "Perceiver". Our INFJ Personality Type is now pretty firmly set in place, and we know the dominance ordering of the four functions.
From 20 - 35 years
We begin to use our tertiary function more frequently and with better success. Our INFJ begins to use his Introverted Thinking function. He continues to make judgments with his Extreverted Feeling auxiliary function, but he also begins to make judgments based on logic and reason, which he works through in his own mind, rather than discussing it with others.
From 35 - 50 years
We pay attention to our fourth, inferior function. We feel a need to develop it and use it more effectively. Our INFJ begins to use his Extraverted Sensing function. He becomes more aware of his surroundings and begins to take in information from others in a more literal, practical sense. He continues to rely on his dominant Introverted iNtuitive function to take in information, but he is more able to use his Extraverted Sensing function than he has been before in his life. Some researchers have attested that the appearance of our inferior functions at this phase of life may be responsible for what we commonly call the "mid-life crisis".
From 50 onwards
From this age until our deaths, we have accessibility to all four functions. However, we use them in a more disciplined, differentiated manner than when we were very young. Our basic Personality Type continues to assert itself, but we are able to call upon all four functions when needed.