In the essay "Symptoms," John Steinbeck discusses the effects that war can have on soldiers. He argues that soldiers are often reluctant to talk about their experiences, and that one of the reasons for this is because they are likely to forget a lot of it as a defense mechanism.
Steinbeck explains this by comparing it to childbirth. He tells us that women have the ability to forget about pain after childbirth because this enables them to have more children. It is a defense mechanism. Similarly, soldiers are able to forget the hardship of war in order to keep living their lives or to fight in a war again.
Steinbeck probably chose this analogy in order to explain the experiences of war in a way that was more relatable to the reader. It is a successful analogy because it fulfill its purpose of explaining what war is like for soldiers.