One of the reasons dogs work so well with humans is that dogs are pack animals, and they are happy to be part of a mostly human "pack."
But when the humans disappear all day, going off to work or school, the dog has suddenly "lost" his pack. This can be a very traumatic event for the dog, and if repeated everyday, very bad for the dog…very neurotic inducing! Just look for some of the YouTube videos showing what dogs do when the owners leave the house!
For that reason, we always try to have at least two dogs – not just one. This way, when we go off to work, they still have their pack and are not abandoned. It's more work and expense to have two dogs, but it's just so much better for them. We don't get quite the "rockstar" welcome when we come home, though they are exuberantly happy to have the whole pack back together. And when the kids and grandkids come over, well, the only thing missing is a moose to chase down!
Dogs are pack animals, with a pack hierarchy, gorge at a kill, and express displeasure with their teeth. Fail to recognize that, and you will have problems with your dog.