Why are There 8 Billion of Us? 

You can find one set of answers to this intriguing question HERE. And among the answers is friendliness! Compared to Neanderthals, Homo sapiens (thats us) lived in larger groups and interactions with other groups were more likely. Thus finding a mate from outside your family was more possible. Genetic deformities are less common in Homo sapiens than in Neanderthals.

(As an aside, I have been told that the invention of the bycycle was the cause of vastly improved genetic health in the nineteenth century.)

Homo sapiens may have had a slightly higher rate of reproduction; their friendliness encouraged the creation of networks which allowed the spread of technologies which got us through changes in the worlds climate. Other humanoid species were less resilient.

This is an interesting article, even if much speculation is involved. And it has the following graph which is frightening!

Based on estimates by the History Database of the Global Environment and the UN. Max Roser, CC BY-SA