Here's a reason: One member of the family lunges at another family member's neck with teeth bared.

My best friends had to re-home their dog after she went after their one year old, who was simply walking past the dog at a distance of about eight feet on the way to go see mom. I know these people intimately, I've known them for years. I had taken care of the dog before, had seen how their son interacted with the dog, and had seen how they supervised their son around the dog. I can honestly say the child was always well behaved around the dog, was never allowed to harass the dog, and the dog was not neglected by these people.

However, this dog was a whiny little prissy bitch who would do shit like piss on the carpet because she didn't like the feeling of wet grass under her feet and refused to go outside when it was raining. She'd growl and snarl if the kid was on the same floor of the house as her, even if he was in a completely separate room in the opposite corner.

That's why you get rid of a "member of the family." Fortunately her previous owner took her back in a heartbeat, but I absolutely would not hesitate to do this, either.

Edit: Yay downvotes! Seriously you guys? Because there couldn't possibly be any reason on the face of this earth to rehome a dog and clearly a dog that almost bites the face off of a small child is a priority over said small child. Also I'm totally making all this up. Only the people in this sub were kind to animals when they were young and no animal ever does anything wrong that requires you to make a heavy decision . /s