Paul Morel has an extremely close relationship with his mother. Mrs. Morel is the quintessential doting mother. That might not sum it up well enough. She's more of a hovering mother than a doting mother. She has four children, but can't quite seem to divide her love equally. For the first part of the novel, her attention was focused on William. Unfortunately, he dies, and the result is that Mrs. Morel is devastated. She is so devastated that she barely pays any attention to her remaining three sons. That is until Paul almost dies too. From there on Mrs. Morel makes Paul the focus of her life, and she is the focus of his.
The downside to such a relationship is that Paul is never truly able to love another woman more than he loves his mom. It's not for lack of effort either. Paul becomes quite close to a woman named Miriam, but it is a friendship closeness, not a lover's closeness. Their relationship goes on for a few years; they do eventually have sex with each other, but then separate. Mrs. Morel does not like Miriam and the fact that Paul is so close to her. Because Paul so deeply cares for his mother, that is likely the reason the two of them never get married and is likely the reason Paul ended the relationship.
Mrs. Morel essentially prevents Paul from stepping into a truly masculine role -- that of being a provider for a wife. He is forever destined to be cared for by her, which is quite emasculating for Paul. The saddest part of it all is that Paul either doesn't realize it or just doesn't care.