Dr. Hendrix says that when a person falls in love, they may appear to be attracted only to their partner's positive traits. Subconsciously, that person is also attracted to their partner's negative traits that are connected to the unfulfilled needs of childhood.
The Imago Theory is that it's not a coincidence you're attracted to your partner. "You lost [happiness during childhood] and you'll never feel fully alive again until you get it back. We think that committed partnership or marriage is the context within which those issues can be restored."