We can't get along for several reasons.
Me, me, me, is more important than you, you and you. IE Selfishness.
We don't usually understand others, who they are, where they come from (their history), why they are as they are etc. Sadly, we don't take the time to find out.
Racial, social, religious, economic, ethnic and geographical differences separate us.
Poor upbringing, not financially so much, but in education and training. Racism, for one thing, is taught, either by direct teaching, or, more often modelled.
There are probably more that I don't care to mention, since this is not meant to be too long an answer.