Why are dogs very protective of their owners?

Dogs are pack animals. In the wild they ran in packs and instinctively they protect their pack. Domestic dogs think of their owners as part of their pack.

The dogs get very attached to one person. That person is usually the person who spends the most time with them, which should be their owners. If the dog feels like something or someone is a threat to their owners, they will growl and a back off warning and if you don't they will snap or bite at them.