difference?

I have been thinking a lot about difference.  Since Orlando, and all the terror since.  Since childhood, and all the terror since.  There seems to be something so fundamentally threatening to us humans about the simple reality that people are different.  In many ways, in every way.  We think differently, we feel differently, we look different.  Our brains are different, our guts are different, our bodies are different.  We each have our own subjective reality that is informed by the infinite variations in our genetics and our experience.  

I know we need to connect, more than anything.  But I am struggling to understand why we seem to require sameness to be close.  When we first meet, it's so exciting to cross the distance of difference.  We thrill to discover the ways we are the same in a sea of uniqueness.  And then when we start to know each other and be known, we start the process.  We pick at each other.  We pressure each other to agree.  We threaten to go away.

Some people require sameness right from the get go.  There is no pleasure in difference, no curiosity, just threat. 

What are we afraid of?  Is it really difference?  Is it maybe the painful reality of our separation from each other?  Would we rather hurt each other than tolerate the uncertainty that comes with being human?  What do you think?