Never tell a peson who's experiencing deep sorrow, i know how you feel..

i receive a txt today and it said never tell a person who's experiencing deep sorrow, i kow how you feel because you don't. Its true, maybe you went through that same situations but you have different emotions. Nobody knows anyone's emotions but themselves alone, whether your a friend or family or really close to that person but you really dont know whats going inside. what is your reactions to this? what do usually do when your comforting other people who is really in deep pain?