"I understand"

Apparently this is the worse thing you can say to someone?
I said this, in an attempt to be kind, and was immediately attacked because there was "no way I could understand unless I was that person".
That's not what I meant asshole!
You try to be nice to someone and that is what you get?
Why do I even bother to talk at all? I can't fucking win either way. I either don't talk enough or I offend someone. What the hell am I supposed to do?