Okay kiddo, you know how sometimes grown ups do nice things for other people, like saying please and thank you or helping them when they need it? Well, some people believe that everyone has the ability to be a good person and do those kinds of things naturally, without being asked or forced to do them.
This is called the theory of good human nature, and it says that at our core, humans are good and want to help each other. It also says that if everyone acted on their natural goodness and helped each other out, the world would be a really great place.
Now, not everyone agrees with this theory because sometimes people do bad things, like lie or hurt others. But those who believe in the theory of good human nature think that these bad things happen because people get upset or scared or feel like they have no other choice, not because they're naturally bad.
So, basically, the theory of good human nature is all about believing that people are naturally good and want to do good things for others. It's a really nice idea, isn't it?