Iona's story in "Misery" would be a harsh, unhappy one even without the death of his son. He spends his life waiting for fares in freezing weather, and when he does pick up passengers, they treat him with brusque indifference. The fact that his son has just died, however, makes the misery of Iona's situation even more stark. He cannot ignore the fact that no one he meets cares about him or treats him as a fellow human being, because now he is continually trying to talk to them. His failure to unburden himself makes the burden heavier and highlights his loneliness in a pitiless light.
The little mare has been out in the cold with Iona, working even harder than he has. Since she has shared his hardships, it finally occurs to Iona to share his grief with the horse. Even though he knows she does not understand his words, he expresses himself in terms she would recognize, asking her to imagine how she would feel if she lost a colt. Denied human sympathy, Iona finally gains some companionship and warmth, including physical warmth, from the horse. This leads him finally to unburden himself, since talking makes him feel better, and the horse gives at least the illusion, and perhaps the reality, of sympathy with his grief.