I have been reading these posts which ponder "why did he marry me?" "Why do they marry?" The reasons obvious, as recounted . . . culture, respectability, & family. More specific . . . money, sex, & services (domestic & secretarial). The last three answer the question of "why are they always entering into instant relationships? Why can't they be alone for more than 45 seconds?"
However, one theory which I have found very useful is the notion of the "self-object." The self-object is the child's blanket or small toy -- held in the hand when leaving the house. The self-object is associated in the child's mind with home & his mother. So when he is away from home & his mother, he is not thrown into an abyss of insecurity because his sense of self is not fully developed & he is so dependent on the mother & the home. So when he is in day care, etc. he holds the blanket to comfort himself when stressed or unsure. It grounds him.
Now, for these personality disordered men, the woman is his self-object. Without her he has no place in the world. It is an obvious mother identification. But a grown-up man needs sex, not breast milk. But like a child, he is drawn to the mother, but he feels suffocated by her because he needs to develop his own independence & identity. And like a child, he kicks the mother/self-object girlfriend around because like a mother he feels she will always love him & never abandon him. They want the girlfriend to render the unconditional love that the mother represents.
The need for a self-object is why they go from woman to woman. Overlapping & lining up. One relationship dovetails right into another. Without a woman as "home" & a person to abuse, they are lost. Like a child feels the mother is part of him, so the PD feels the woman is an extension of him. So they cannot be alone. And they are really very fragile beings, although bullys underneath. They pull on women's maternal instincts. So many women say that they knew this guy was weak & fragile psychologically . . . however, these same women are sometimes hospitalized by the physical violence of these men.