You may be interested in understanding how we came to the conclusion that certain types are ideal for each other. Our type pairing is a result of combining observation, research, and understanding of Jungian psychological type. Observation and research of married couples shows that there is a definite trend in types that are attracted to each other, and in type combinations that have the longest lasting relationships. Our understanding of psychological type helps to see that these types typically have the same dominant function, but with a different attitude.
In Jungian terms, "functions" refer to the four core traits: Intuition (N), Sensing (S), Feeling (F), and Thinking (T). The term "attitude" refers to the direction of the function, i.e. Introverted (I) or Extraverted (E).
So, for example, a person with a personality type of ISFP has a dominant function of Introverted Feeling. More specifically, Feeling in the dominant function, and its attitude (or direction) is Introverted. A person with this personality type is likely to be most attracted to, and fit best with, a person that has Extraverted Feeling dominating their personality. Extraverted Feeling dominates the personality types ESFJ and ENFJ. We therefore determine that the ISFP's natural partner is the ESFJ or the ENFJ.
Our natural attraction to people who share our dominant function, but who use it in a different direction works very well for us. We not only flip-flop the Introverted or Extraverted trait, but we also flip-flop the Judging or Perceiving trait. In this way, the partner that we choose for ourselves will have a very different approach to dealing with the world. If we are laid-back and indecisive, our partner will be structured and decisive. If we are reserved, our partner will be outgoing. For all of our apparent differences, we will share a common vision of what's truly important in life.
For people whose personality types are dominated by Decision Making functions, (i.e. Thinking or Feeling), their ideal partners will include both Sensing and Intuitive types. Many people have problems communicating effectively with people who do not share their same preference for Information Gathering. So, if you have a very strong preference for Sensing or Intuition, you will need to give the personality type with the same preference a higher value as a likely natural partner. For example, an ISFP who strongly prefers Sensing will work best with an ESFJ, rather than an ENFJ.
Although we believe firmly that this model works very well to help in finding and maintaining healthy relationships, it is important to remember that it's just a tool. We offer guidelines to help you understand the kinds of things that you value in a relationship, rather than guidelines that you need to follow strictly. Two well-developed individuals of any type can make a relationship work. And work is a key concept here! There is no such thing as an effortless relationship. Don't use this model as an excuse to dump your relationship.