Human nature is a very complicated topic to discuss for there are many different angles to approach. However in my opinion, the factor that affects human nature the most is the personal experience of that person. These past experiences determine how well you view life, your personality, etc and can very well explain the nature of that specific human.
What usually affects your behaviors the most is during childhood. You learn basic actions and as you get older, you learn different skills and also develop your interests. However at this point of your life, it can also create bias and it can make you develop some very unnecessary thoughts about other things such as racism, religious tolerance, etc. Although you can develop these things when you are a teenager or older, your childhood is a very crucial time of your life for it is the first time you actually experience these types of thoughts and the first time you can make a decision of where you might lean towards to.
Another huge factor that can happen spontaneously is when traumatic experiences become a part of your life. When someone close mysteriously died or you just got dumped by your first girlfriend, this can trigger something deep in your mind that can have a long-term effect on you. You can feel as if the world is against you and that will definitely change your human nature dramatically.
As I said before human nature is ever-changing and is mostly self-interested for you experience it yourself. So if your view of mankind is a dreary one, you can still change it but it will require you more work for you have many directions that you to decide on. That is why your parents try to steer you in the right direction in their...