To best understand this question, it's important to understand what caused a separation between Harpo and Sofia. Early in their marriage, Harpo gets upset that Sofia does not obey him. Many women in this time period, the 1930s, would generally obey their husbands (and fathers) without disagreeing. Most women were not expected to think independently or assert their opinions. Harper tells Celie:
I want her to do what I say, like you do for Pa … When Pa tell you to do something, you do it … When he say not to, you don't. You don't do what he say, he beat you. (62–63)
Sofia was different from many women in her time. She did not simply obey, and she did not appreciate her husband's (Harpo's) expectation that she obey all of his instructions:
I'm getting tired of Harpo … All he think about since us married is how to make me mind. He don't want a wife, he want a dog. (64)
Here, Sofia asserts that she is a human with a brain of her own. She is not an animal that needs to be trained.
Harpo ultimately beats Sofia, and he is hurt by Sofia as well. She is a strong woman, mentally and physically, and she does not simply sit still and let him beat her. Sofia is greatly upset by his decision to beat her, and their relationship is immensely damaged. Both Harpo and Sofia move on and find new partners to date.
Sofia and Harpo remain separated for most of the book. Sofia believes that Harpo is determined to be selfish and and domineering.
Towards the end of the book, Harpo changes. He demonstrates selfless care and concern for another human being:
After [Celia] left, Mr. ______ live like a pig. Shut up in the house so much it stunk. Wouldn't let nobody in until finally Harpo force his way in. Clean the house, got food. Give his daddy a bath. Mr. ______ too weak to fight back. Plus, too far gone to care … Harpo went up their plenty nights to sleep with him … all cram up in a corner of bed. Well, one night I walked up to tell Harpo something—and the two of them was just laying there on the bed fast asleep. Harpo holding his daddy in his arms. (224)
When his father is deeply depressed, to the point of illness, Harpo shows him love and kindness. The compassion that he shows for his father causes Sofia to remember why she loved Harpo. Their relationship is restored:
After that, I start to feel again for Harpo … And pretty soon us start work on our new house. (224)
Harpo's choice to serve and love another person draws Sofia back to him. He stops worrying about whether or not people are serving and obeying him; he simply shows his care and concern for his father. His growing compassion for others causes Sofia to love him once again.