Remembering that people's lives progress and change when you're not around them and you really only know who a person was the last time you happened to see them and you talk to someone you haven't seen in a while and they're not quite the same person as before, they merely resemble the person you remember in your head and you're not the same person they have a conception of in their head because there is no permanent self and no objective viewpoint by which one person can be seen as the same person by multiple observers.