We all fear. It's an emotion that grabs and grips us with a hold that is strong and controlling. Fear rises up in us when we find ourselves in dangerous situations, when we experience pain, evil, lack of control, persecution, and even when we imagine the possibility of such things. Fear is an emotion that stirs us up and we respond. Sometimes fear will prompt an appropriate and positive response or action, like fleeing danger, relieving pain or overcoming evil, but all too often we fear things over which we have no control, influence, or ability to make a difference. In such cases the only thing fear will do is trouble our thoughts and distress our spirit and create in us a stronghold of negative emotion. That will lead us away of from confident trust in the Lord.
"Fear not" is commanded by God throughout the Bible. God speaks those words directly, through His prophets and His angels. Jesus gave this command during His earthly ministry, and He repeated it again in His revelation to John.
The command to "fear not" is always accompanied by reason and encouragement.