If you really think about it dogs are relatives of wolves. Wolves travel in packs and sometimes kick members out if they're too weak, or a different color, or a runt (also falling under too weak).
Dogs on the other hand, have been bred so much that they don't really "kick out" pack members. Although they are still pack animals, they will accept a cat, a human, a baby, or even other different dogs into their pack.
Not to mention many dogs were bred for certain pourposes therefore some were bred to work in what we call "packs" and some were bred to be herding dogs that tend to work alone or in a "pack" of two in some rare cases.
Also companion dogs were bred to put up with being alone with their "owner" or accepting other dogs, thus they have no Pack judgments.
SO technically dogs are still PACK animals but they don't judge their pack as much as Wolves do, due to many many many years of inbreeding, and crossbreeding, they have also been domesticated making some of the oldest breeds, some of the least likely to take to a pack.
Also, Foxes are pretty much loners, and some people believe some breeds of dogs were bred from the fox and the wolf, thus breeding out a lot of the pack mentality that straight descendants from wolves (for example a german shepherd/timber wolf) may have.
So, breeds vary depending on history, but we don't know everything as humans and we can only assume that that is what dogs aim for is forming a "Pack"
If you have any questions on this please ask me.