Something to consider: “Is it possible, in the final analysis, for one human being to achieve perfect understanding of another? We can invest enormous time and energy in serious efforts to know another person, but in the end, how close can we come to that person's essence? We convince ourselves that we know the other person well, but do we really know anything important about anyone?”
Haruki Muramaki, The Wind Up Bird Chronicle
I suppose it depends on what is meant by "important." Perfect understanding may be a shared feeling of unity in spirit and interests. Couples aim to achieve this, but I bet luck is all it takes. And do the variables of sex, age, race, nationality, and education as examples figure in the equation? Pretty deep for the first day of September, my favorite month.