system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are dogs so nice? Why do they have an innate tendency to love and play with people/animals?
gpt
Two reasons:  
1. We've bred them that way. Dogs are the result of between 15,000 and 100,000 years of breeding by humans.  
2. They started out as pack animals, and in breeding we've only increased this instinct. Knowing their position is really important to a dog. Rising up in 'rank' and being the leader is certainly important to them, but inhabiting a safe position is even more important, even if it is the bottom one. So in a normal human - dog relationship, the dog will perceive the human(s) as socially higher up the ladder and therefore to be obeyed and appeased.  When dogs bite other people, they will perceive that victim as a threat to 'their' pack. When dogs bite their owners, it is often in defense because their owners are abusive, or because they perceive their handler as weak and challenge their position. Leg-humping or leash-pulling are less severe symptoms of a dog who thinks he socially outranks somebody.  Striking a balance between the two extremes - being an authority to the dog but not abusive - is not so hard but you'd be surprised how many people get it wrong.