Why do they say "You marry your mother...or your father"?
When we are first attracted to a person we have a conscious experience of liking things about them such as their looks, their intelligence, their sense of humor or a shared interest/hobby/passion. Maybe we just like the way we connect or the way we feel in that person's presence.
But, often, when the relationship gets past the "honeymoon phase", relationship dynamics develop that have their roots in the unconscious. Some aspect of our struggles with one of our parents (or occasionally, with a sibling) gets stirred up. We may unconsciously bring out that quality in our partner, or they may carry it in a way that initially went unnoticed or didn't emerge.
For example, let's say Suzanne and Bob meet at a party, are attracted, find they share values and interests and get together. They have fun, get more and more serious and eventually get married. As their life becomes more like the family life they grew up in, with the demands of balancing work, recreation, cooking and cleaning, friendships,etc., they begin to feel certain irritations or disappointments with each other. They are not aware that the intensity of these feelings have their beginnings from before they ever met.
So, Suzanne, having had a dad who was always emotionally absent starts to resent Bob for having to work late at times and for wanting to go hiking with his best friend often on weekends. She feels emotionally abandoned and hurt. Her hurt turns into anger which comes out in snappiness and irritability with Bob.
Bob, on the other hand, grew up with an alcoholic, angry mother and is very sensitive to Suzanne's anger at him, although it doesn't look like his mother's, of course. But it is an area of vulnerability for him, and he doesn't know how to deal with it. He does what he did with his mother, which is to withdraw. Naturally, this makes Suzanne feel even more emotionally abandoned, hurt and angry.
They are both unaware of the reason their "buttons are getting pushed", but they are now caught up in a painful distancing cycle. When they come to therapy they are frustrated and unhappy but are helped by developing an understanding of their underlying family of origin issues and of the cycle in which they have gotten caught. As they unravel this and they are helped to communicate their real feelings and to learn to listen carefully to each other they are able to re-connect and renew their relationship.
(The above persons are not real clients, but a fictional construct, created to explain this common type of pattern seen in couples).