You are asking, "we are usually attracted to being intimate with one person in particular."
It has a psychological reason. You are brought up by a mother, by a father; if you are a boy, you start loving your mother and you start being jealous of your father because he is a competitor; if you are a girl, you start loving your father and you hate your mother because she is a competitor.
These are now established facts, not hypotheses, and the result of it turns your whole life into a misery. The boy carries the image of his mother as the model of a woman. He becomes conditioned continuously; he knows only one woman so closely, so intimately.
Her face, her hair, her warmth — everything becomes an imprint. That's exactly the scientific word used: it becomes an imprint in his psychology. And the same happens to the girl about the father.
When you grow up, you fall in love with some woman or with some man and you think, "Perhaps we are made for each other." Nobody is made for anyone. But why do you feel attracted towards one certain person?
It is because of your imprint. He must resemble your father in some way; she must resemble your mother in some way.
Of course no other woman can be exactly a replica of your mother, and anyway you are not in search of a mother, you are in search of a wife. But the imprint inside you decides who is the right woman for you. The moment you see that woman, there is no question of reasoning. You immediately feel attraction; your imprint immediately starts functioning — this is the woman for you, or this is the man for you.
It is good as far as meeting once in a while on the sea beach, in the movie hall, in the garden is concerned, because you don't come to know each other totally. But you are both hankering to live together; you want to be married, and that is one of the most dangerous steps that lovers can take.
The moment you are married, you start becoming aware of the totality of the other person, and you are surprised on every single aspect — "Something went wrong; this is not the woman, this is not the man" — because they don't fit with the ideal that you are carrying within you.
And the trouble is multiplied because the woman is carrying an ideal of her father — you don't fit with it. You are carrying the ideal of your mother — she does not fit with it. That's why all marriages are failures.
Even in marriage, the basic reason for which you wanted to have the relationship is not fulfilled.
You are more alone when you are with your wife than when you are alone.
To leave husband and wife in a room by themselves is to make them both utterly miserable.
OSHO