Probs because the puppy was taking food from him, like how he was taking money from his father. Maybe ...
he gets no affection at home or from nonexistent friends so when someone shows their true vulnerability & love (the dog) he doesn't know how to process it and feels weird and awkward about it. it han't happened with protag so far because their friendship is still shallow on some levels. it will probably become an issue when the protag starts falling for him for real.
He pities his past self ..seeing it in the dog
Oh my God now I see it
Why was he disgusted when he saw the dog? That cute little puppy