Separation naturally leads to judgment. Since we're "different" I can form an opinion about how you do things vs. how I would do them. Making distinctions then lets me label things as good or bad, right or wrong, etc.
Judgment can naturally progress to fear. Seeing separation and forming ideas about how I'm right and you're wrong can turn into me fearing that you getting what you want is dangerous to me. We all want to be liked and we want to be right. If our opinions differ, we fear what the opposite holds for us and our way of living.
This is how we progress from a state of separation to one of fear. Over time, these become pathological (anxiety disorders are an example). You can stop the progress of these patterns by starting to recognize when you're in them. Notice when you sense separation, are judging others, or acting out of fear. Just noticing is the first step to breaking free.