Complex sentence: The biggest problem with this comes when humans forget that a dog is a dog, when dogs start to cross over into the realm of "baby" or "sibling" it becomes really hard for humans to supply basic needs to their dogs.
Split into simpler sentences: The biggest problem with this comes when humans forget that a dog is a dog. When dogs start to cross over into the realm of "baby" or "sibling" it becomes really hard for humans to supply basic needs to their dogs.