My family has always gotten shelter dogs. Usually full grown, frequently with some history of mistreatment in their past. We are animal people and love to spoil our pets. Quite often the dogs we get are terrified of something or another from their past, and as such they require a whole lot of patience care and love. If your willing to take the time to build that trust though a dog becomes fiercely loyal,exceptionally trusting of the family, and caring protectors from outsiders. I've had a dog (Lab/shepard mix we think)that when we first got it was terrified of anyone picking up a shoe(it had been beaten pretty badly) who, over time, and with patience, started bringing me my shoes in the morning with his leash draped over them for his morning walk. Same dog also tackled and held my sisters boyfriend pinned to a corner until dad and me came home after he slapped my sister. His lips stayed curled back, with teeth bared and a very audible growl, until my dad grabbed the kids collar. Then the dog went and got his leash and came up to me, tail wagging like a puppy, wanting to go for a walk, while my dad had a very serious discussion with a very terrified 15 year old guy that ended with him leaving the house airborne. The dog got a steak with his dinner that night :D
My folks current dog came to them with burns all over her...I've finally established enough trust that she doesn't hide under the porch every time I light up out back with her. Only took three years.