Human nature refers to a set of attributes that people tend to have which naturally differentiate us from other animals, including the way of thinking, feeling and behaving. An example is our social nature because we regularly interact and cooperate with other individuals in order to achieve common goals. Being socially involved is in fact beneficial for our mental health. But what are the underlying mechanisms that determine people to initiate, develop, and maintain social relations? And more importantly, is there any limit to the size of human social networks?