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2. Overstressing
If you don't handle stress well, it's likely your dog won't, either, suggests a study in PLOS One. By evaluating the personalities and cortisol levels of 132 people and their dogs, researchers were able to determine that pets living with optimistic or open-minded humans were better at adapting to new and challenging situations than dogs whose owners tended toward anxiety or depression. "Dogs have become highly capable of reading our emotions," says lead study author Iris Schöberl of the University of Vienna. A pet's anxiety could lead to inflammation and higher cortisol levels, which could affect her heart, kidneys, and liver, says Ward.

Pet Rx: Some signs that your pup is going through a rough patch include decreased interest in activity, pacing at night, and a spike in appetite (dogs stress-eat, too), according to Ward. Mention these to your vet, and consider whether there's something going on in your life that could be trickling down to your dog, Schöberl says. Find ways to address your issue, perhaps by consulting a therapist.