It could be that everyone has a story. The grumpy old man had a past full of experiences. By listening to others' stories, we can gain understanding and empathy. In the case of the children and the old man, this mutual understanding made their relationship better and lessened the old man's grumpiness.
One possible moral could be that appearances can be deceiving. Just because the old man seemed grumpy on the outside, he had a softer side and interesting stories to tell. People should look beyond the first impression.