the audience is supposed to believe that hachi, that's the dog's name, walks with nakadai as he gets on the bus to go to work, walks home by himself, and then walks back to the bus station to greet nakadai as he is coming back home. pic related shows hachi snapping at his hand to get at the treat, meaning that everything the audience was supposed to believe in shortly before falls flat immediately afterwards because nakadai is unable to get the dog to walk home without goading him with treats. the logical conclusion is that there is no way the dog was able to do the things portrayed in the scenes before without assistance.