Dogs are more similar to humans than once thought. They suffer from the pangs of jealousy just like us. They do not like to share the affections of their owners. When they see their owners giving attention to another dog it brings out strong emotion in them. It was once believed that dogs were only capable of having primary emotions such as anger and anxiety now research has shown that they have secondary emotions such as guilt, jealousy, embarrassment and shame. Dogs show a strong dislike to unequal treatment. They do not like to see another dog getting preferential treatment over them. Dogs understand fairness. Dogs have the hormone oxytocin like us humans which has shown to bring out love and jealousy in humans.
Dr Range from the University of Vienna headed a series of experiments with dogs who knew how to give their paw. The dogs gave their paw weather they got a treat or not. All changed when one dog was given a treat for giving the paw and the other not. The dog who did not get the treat would not give the paw eventually and would not look at the researcher. You should treat all your dogs with equal care and attention. Even if you get a cute fluffy new pup make sure you give as much attention to your old dog. Experiments have shown that dogs are more sensitive to fairness rather than equity.