#14 Not Socialising The Dog
Children need to be taught socialisation so they may learn how to interact with the outside world.
Likewise, dogs need to be socialised too. They need to learn how to approach strangers, be around familiar faces and react to intruders or suspicious people. This also applies to other animals as they are all new to the pup.
Not helping the pet through this process can result in the dog getting scared, angry or even aggressive towards others as it does not know how to gauge the situation, treating everyone and everything as a threat, towards which they can be hostile.