When you have been friends with someone for a long time, you start to pick up little things about them, little facial expressions, certain ways that they talk or move their hands. Sometimes it even gets to the point where you subconsciously start mimicking those little things.
and that is when it hurts, when they are gone and you catch yourself doing something that has come so natural because you are so used to having them around. When you can hear their voice in in your head because you know exactly how they would say it. When you feel like you are telling a story with your body and the way you act but it is only one half of the story.