Because when people or animals are younger it seems cute and we think it is just play, yet, as the animal or person grows, the 'playing' can become rough and lead to actual fights since they are usually stronger and isn't really considered 'playing' anymore, as well as during different ages, people and animals act differently. You really have to look at all the aspects and views. Yet, some people may have different views, as saying that it is always playing, or some people think that its always fighting. Yet, how would we know? We don't.