Why does the old farmer in The Silver Sword have a gleam of sadness in his eye?

In the book The Silver Sword by Ian Surrailier, a farmer gives refuge to the main characters after discovering them hiding on his farm. The farmer, Kurt Wolff, has a sad gleam in his eye due to the death of both of his sons during World War II. His wife tells Jan, one of the child refugees, about one of the sons, Rudolf, saying that Jan would have liked him because they were both good...

In the book The Silver Sword by Ian Surrailier, a farmer gives refuge to the main characters after discovering them hiding on his farm. The farmer, Kurt Wolff, has a sad gleam in his eye due to the death of both of his sons during World War II. His wife tells Jan, one of the child refugees, about one of the sons, Rudolf, saying that Jan would have liked him because they were both good with animals and the family dog reacted to Jan the same way he had reacted to Rudolf. In fact, when the children leave the farm, they find the dog, Ludwig, has hidden inside their provisions and stayed with them.

While the death of his sons left farmer Wolff with much sadness, learning about Rudolf had a positive effect on Jan. For most of his life, Jan had been a thief, even facing detention for stealing from American troops. At the end of The Silver Sword, it is revealed that Jan gave up his criminal ways and focused on his love of animals, often being called to help the sick and injured.