G.E. Moore says, here is a hand, and here is another hand, therefore, hands exist.
I have two hands. They are capable of grasping and pointing and manipulating things around me. Genetically, humans have hands. However, other organisms have hands. I have two eyes that point forward and give me three dimensional sight. They are capable of perceiving colors between infrared and ultraviolet. Genetically, humans have eyes, however, other organisms have eyes, and some organisms have much better eyes than humans. I have this body capable of bipedal locomotion. Genetically speaking, humans have bodies that are capable of bipedal locomotion, other organisms are capable of bipedal locomotion but very few use it as their primary method of transportation. However, not all humans are capable of bipedal locomotion, due to any number of factors. This goes for the previous two things as well. I have these emotions and perceptions and ideas. I believe in concepts and I think about things. I have the ability to make choices and understand the risks and outcomes. As far as I know, genetically, humans have these things. This is not something I can directly perceive, but it is something I have inferred. However, it seems that most animals are capable of these things as well, based on the same perceptions I use to judge the presence of these features in humans. Granted, other organisms don't seem to possess the same complexity of mental faculties, but I can't know for sure.
I have this idea about what being "human" means. And it seems that I possess whatever qualities make one human, and thus I think that I am human, and I think that those organisms with similar traits I interact with are also human. Through my interactions with humans, I have observed that there are certain behaviors that seem to be preferred. By preferred, I mean that some feelings occur as a result of an action that would be chosen over other feelings if such a choice is possible. Universally, humans seem to choose pleasure over pain. Even those known in human society as masochists, who choose to have pain inflicted on themselves, do so because it gives them pleasure. When I interact with the world, I prefer to have interactions that result in pleasurable feelings. For instance, when I hold a conversation with a fellow human, I would like to leave this conversation feeling happy rather than angry.
When human A interacts with human B, both participants would prefer to leave the interaction with feelings of pleasure. They both have the option to behave in a way, P, that invokes pleasure, or a way, Q, that invokes pain. It can be understood that if A behaves P and B behaves P, then P will be mutually invoked, therefore, both participants leave the interaction with feelings of pleasure. The converse is true, if A behaves Q and B behaves Q, then both participants will leave the interaction with feelings of pain. (Keep in mind, this is true even of masochists, because masochists still ultimately seek pleasure.) If all humans prefer pleasure over pain, then it makes no sense for A and B to both behave Q. It makes perfect sense for both to behave P, as it is mutually pleasurable. However, if A behaves P and B behaves Q, then B still gets pleasure out of the interaction, even if A does not. So, what is B's motivation for behaving P, if he can receive pleasure even if he behaves Q? The short answer is that eventually, A will cease behaving P towards B, so that a pointless QQ interaction will occur and the participants cease the interaction. So, in order to continue receiving pleasure, the best course of action is to behave P, so that mutually beneficial PP interactions can continue to occur.
Treat others how you want to be treated? Not exactly. Treat others in a way that invokes a pleasurable experience, so that mutually pleasurable experiences may continue to happen in an effort to maximize pleasure and minimize pain.
Why, then, do some people insist on behaving Q?
Because they're dicks.
