Except everyone already mentioned there is also another "trick", which could help sometimes. On pics 2 and 3 the dog is on the first plan but all in shadow because unfortunately sun is behind him. In such a case if you can't recompose the shot, you should put more light onto the subject you shoot, in this case - on dog. You can do this by two ways:I totally agree. In the pictures were the dog doesn't have much pop, you don't have any light on the dog's coat. you need to reflect light into his coat, use a flash, or overexpose your image.
1 and better by reflecting some sun rays onto the dog (but I assume you don't walk for walks with a dog with an assistant holding some silver foiled panel) and
2 and worse use a flash, even the one built in camera.
This way you could avoid somehow such a difference in lighting between first plan and background. This funny btw and not natural to use additional source of light in such a sunny day, but this is exactly how it works