The answer is to be found a little way into the book when the reason for Mr. Tom's slightly bizarre behaviour is revealed. Mr. Tom used to be married, but tragically his wife and their newborn son both died of illness, leaving him absolutely bereft. The way he dealt with his grief (or rather didn't deal with it) was to cut himself off from the world and to become a rather gruff individual who kept himself to himself. What is key to the story is how Will, the evacuee from London, manages to open up Mr. Tom and help him to process his grief, in a sense becoming the son that he had lost all those years ago. Thus this novel is a rather touching tale of how two unlikely characters are able to support and help each other through their own difficulties to process their own traumas.
mr tom becomes a recluse because his wife Rachel and his own son died when he was small. so that he didn't have anyone to show kindness.