It's not "I know how you feel"

 It's

" I know what you feel"

If you ever try to comfort someone, think first of what you'll say. Because sometimes, we are being insensitive by saying the wrong words, specially when we've never been in his/her shoes. So my thought is, saying "I know what you feel"  are for those people who only KNOWS what it feels and "I know how you feel" for those people who've experienced the same tragedy in life.