How often have you heard someone say to you "I understand"? Perhaps you are someone who uses this as a way to establish a rapport with another. Unfortunately this short phrase has the opposite effect, it destroys rapport.
Each of us is very different - we see, feel, hear, and process things in varying ways. This is due to our thoughts, attitudes, beliefs, emotional awareness, adaptability, etc., etc. Additionally, we never know what else is influencing the person we are speaking with. Our personal life has a direct impact on our emotions and again for each of us this is different.
Try to avoid using this phrase, instead replace it with an open question to allow the other person to explain how they are feeling and what they are going through. "How are you coping", "that must be hard for you", and "I'm listening" are all valuable replacements to "I understand".