Sadly, what is the main reason why Bernese mountain dogs have such a short life expectancy?

Unfortunately, Bernese Mountain Dogs have one of the shortest lifespans of any dog breed, if not the shortest.

While the American Kennel Club says that the lifespan of the Bernese Mountain Dog is 7-10 years, most sources put their lifespan at more like 6-8 years. A Swiss study put the median life expectancy of Bernese Mountain Dogs in Switzerland at 8.4 years. A Danish study found the average life expectancy of Bernese Mountain Dogs to be only 7.1 years.

No matter how you slice it, the lifespan of the Bernese Mountain Dog is significantly shorter than the average lifespan of all dog breeds combined, around 10-12 years. While large and giant breed dogs tend to have shorter lifespans than small dogs, Berners have potentially the shortest lifespan of any dog breed.

The biggest factor in the short lifespan of the Bernese Mountain Dog is their high rate of cancer. Around half of all Berners die from cancer, which is a significantly higher rate than most other dog breeds. On average, fewer than one out of every four dogs die from cancer.