Why is Steinbeck puzzled by soldiers reluctance to discuss their battle experiences?

From the text:

If they had been reticent men it would have been different, but some of them were talkers and some were even boasters. They would discuss their experiences right up to the time of battle and then suddenly they wouldn’t talk any more. This was considered heroic in them. It was thought that what they had seen or done was so horrible that they didn’t want to bring it back to haunt them or their listeners.