Earlier quoted context omitted.
Cows are fairly peaceful animals that naturally live in a herd and have been domesticated for thousands of years (people can get killed by cows, but it's rare). Bears are huge ferocious animals that typically live solitary lives and will kill you and your entire family while hardly breaking a sweat. Keeping polar bears "like cows" sounds like some good comedy sketch material.
Counterpoint: we did it with wolves.
True, but this took thousands of years. We could domesticate descendents of polar bears, perhaps. But those would no longer be polar bears.
Also, injuries and even deaths from dogs are still quite common, and polar bears (and their hypothetical domesticated descendents) are a lot stronger.