For the most part, these letters concern Harpo's aching unhappiness because of the fact that the only role model he has for being a husband and a man is that of his indolent father, Albert. Unfortunately, Harpo thinks that he himself is a failure — simply because he can't beat Sofia, like Albert beats Celie. Harpo doesn't realize that his role model is wrong — not Harpo himself — and as long as Harpo is married to Sofia, he will never be able to treat her as Albert treats Celie. Sofia is a strong woman; she won't stand for it.
For that reason, then, Harpo tries unconsciously to be big — big and strong and powerful — like Sofia. But that too fails; all he gets is fat. He becomes the butt of many jokes and gets such a pot belly that he looks pregnant. Yet, in his case, he is not pregnant with anything positive; he is bloated with confusion and resentment and self-pity.
Harpo doesn't realize that he is far luckier than his father. He and Sofia truly love one another — despite their constant fighting. Between Albert and Celie, there is no emotion. Therefore, in a marriage like Harpo and Sofia's, where there is love, there is also room for variation in roles, which is why Harpo and Sofia are able to divide the cooking and the cleaning. In a loveless marriage, such as Albert and Celie have, there is no room for variation. Celie must do what she is told, and Albert does whatever he wants.
Celie tries to explain to Harpo that Albert and Shug are in love and that their relationship works because of their love for one another, but Celie points out that they are not married. A wife and a mistress always have different duties. Harpo still doesn't understand.
