Sometimes we get an abrasion. A rug burn. It hurts for a day or two, we put some antibiotic ointment on it and maybe a bandage and we go on.
Every once in a long while we take a tumble that leaves a deeper wound. We sit, stunned, watching ourselves bleed and we might even cry. This time we need to stop the bleeding, clean the wound and use a more serious dressing on it. Maybe even gauze wrapping to keep it in place. Now it hurts for a long time, affecting everything we do until we are significantly along the road to healing.
Losing co-workers is like this too, You have that layer that comes and goes and that you don't notice much. You have some that make you pause when they leave but you get over it. And then there's a few that go away that really affect you and make you re-learn how to work without them. Those are the ones you really miss. You heal eventually, but there's always a scar left behind to remind you of your loss.