It is important to discuss and define the concept of human nature. Human nature it is central to secular thinking because secular thinking relies on the authority of mankind rather than the authority of a god or gods.
Human nature can be defined as traits that are common to most human beings. Human nature has been relatively constant for thousands of generations, ever since mankind finished evolving into a conscious creature. Human nature is founded on the physical and mental nature of mankind and the fact that we are more similar than we are different. It is these common features which make us think and feel similarly in similar situations across the species and across the relatively recent history of humanity.
Other animals share similar traits which identify the nature of a particular type of animal. Cats share a set of common traits that we can call cat nature, or the nature of cats. Similarly with dogs, snakes, chimpanzees, etc.
But what makes human nature notably different is that mankind is conscious and self aware. It is this self awareness that gives human nature a much larger range than animal nature.
Human beings of all races and throughout recent human history have much in common. We are more similar than we are different. For this reason we can and do generalize about people and call it human nature.