What Drives People Together and What Drives them Apart

Reviewing Dungeon, no Deal... got me thinking that I need to write a more generic article about things and forces that either drive people together or drive them apart. 

Some people have tendency to favor their families and then distant relatives over others. Sometimes this mindset extrapolates into nationalism where they favor their own nationals over others, often falsely believing that citizens of their country are more related to them genetically than foreigners. 

Such people might then think that people who are not like them would always treat everyone the same, regardless of their creed or origin. After all this would be polar opposite of what they themselves do.

However, reality is different. People always unite on some common ground. The difference is that sometimes this common ground is obvious and clearly identifiable. Other times is ambiguous and obscure. Common ethnicity, looks and blood ties are something one can often see with their eyes. 

On the other hand, common interests, hobbies, similar personalities and so on are much less obvious for an average observer. Yet these things connect people none the less. It's like a gravity force: its invisible, but its strong and the only reason we walk the Earth and not float in the sky.