Bernese Mountain Dogs are affectionate and easy-going, making them a great addition to households with pets, including cats. However, because theyre so large with puppyish energy, younger dogs might accidentally injure cats. You may need to monitor Berners when theyre with cats during their more playful years, up to about 4 years of age. Are Bernese Mountain Dogs aggressive? No, Bernese Mountain Dogs arent aggressive, even though they are big dogs.