Dessa is Dominick's first wife, and he never gets over her. After they split, he obsesses over the things he did wrong during their relationship, like that time he had a vasectomy after their infant daughter died, and that time he raped Dessa when they were still dating. Dominick reacts to these things in the same dispassionate way Anakin Skywalker reacts to chopping off someone's head, like, hmm, I shouldn't have done that.
One night early in their relationship, Dominick forces himself on Dessa against her will. Prior to this, she's told him that he is "the only guy who can make her feel safe" (19.41), which only makes his violation of her trust that much more, um, violating. "What about me?" (20.146), he asks when she says she's not in the mood. That seems to be Dominick's motto for life: What about him?
For some reason, they make up (Dominick doesn't give us specifics) and eventually marry. Dominick and Dessa have a daughter named Angela who soon dies of sudden infant death syndrome. Her death hits them hard. Dessa becomes a "zombie" (14.8) after losing the baby. Dominick reacts by getting the ol' snip-snip, without telling his wife, because he can't bear to go through that again. When he finally admits it, she leaves him, taking the dog, Sadie, with her. It's like a country song.
She moves in with Danny Mixx, an "ex-hippie type" (3.65) who has his own pottery business. Danny's actually really nice, which makes Dominick even angrier. Danny expresses his sympathy to Dominick when Thomas dies, but this sends Dominick into a rage (just like everything else does to him).
After Thomas dies, Dominick starts helping Dessa with children's hospice, holding sick kids, which helps them both reconcile their own daughter's death. Then they get remarried. We don't get to see why Dessa forgives Dominick for all the horrible stuff he's done, or why she thinks he's different, we just find out they remarry in the end. Like Ralph, we rarely get any insight to Dessa aside from her relationship to Dominick.