It can be tough watching your pooch age, especially if the process includes changes in personality. In some cases, dogs who’ve been easy-going since birth become hostile almost overnight. And as stressful as it is for you, it’s just as bewildering to your dog.

Big mood swings often have medical causes — like painful arthritis or a systemic disease. Blunted senses could make your dog startle easily, which could prompt a snarl or a nip. Even canine senility or the rare brain tumor can cause unnerving behavioral changes.

So start by scheduling a veterinary exam. Then make some changes at home to keep everyone safe. Be sure your dog knows when someone approaches. Note if he welcomes or resists your touch in certain places. You may even need to keep young children, other pets and strangers away from your canine curmudgeon.