I think it's largely our collective nature. Without the combined collective knowledge from other humans we would be unusually intelligent apes living in the forest. Seriously, without others we couldn't even build a decent dwelling, we would likely seek shelter from the rain under a banana leaf.
Through complex written and spoken language we have learned to share our knowledge with everyone else, language is essential to human progress. Although progress may be our demise, but that's another post.
List what you believe separate humans from animals.
