Telling someone you understand may not be the best thing to say.
There is no combination of two words that I despise more than, "I understand". People say this all the time, and don't even think anything of it. I am definitely guilty of saying "I understand" before, but it never dawned on me how much I couldn't stand to hear it until about two weeks ago. The two words seem so simple and seem like something that would be completely fine to say, but to some people they mean way more than you might think.
Hearing someone say, "I understand" after telling them anything can be very frustrating. You want to scream but you can't because they probably did not mean any harm by saying it. Instead, you're left with the internal turmoil because the words make your stomach churn but you haven't addressed it. These words are so simple but can mean so much to a person.
I do not believe in saying "I understand" after hearing anything unless it is a fact or direction. No matter what, I don't think that anyone will ever understand how a person feels or what they are saying because most of the time those two people did not have the same experiences.
When someone is angry about something, chances are you have been angry like that before too, but you probably did not experience the exact same thing. Even if two people did experience the same thing, you will never truly "understand" how a person is feeling because everyone experiences things in a different way. Everyone has different emotions toward different things or stronger emotions than others.
Sometimes it can be hard to think of anything to say other than something along the lines of "I know, I understand." I know I have done this before without ever thinking about it. Now that my perspective has changed, some things I would rather hear instead of "I understand", would be "I'm sorry", or even just a simple "that sucks". These words seem simple and some may view them as coming off like they don't care.
After realizing that I could never truly understand what a person was experiencing or feeling, it felt like saying "I understand" showed more of a careless attitude. I believe you can understand what someone is saying, but you can never truly understand how they feel unless they tell you straight up.