Is it bad that I am "too nice"?

I think it's great that you like yourself for who you are. I also understand your frustration with trying to explain your personality with people who don't understand. You are a nice person, you shouldn't have anything to worry about or feel bad about, or want to change for that matter. If you want to disagree with someone it's not a big deal. I think that the reason feel or say that you are being "harsh" or so "upset" when you argue is because they are not used to having you do it. It's new to them. So to see you disagree at all would seem out of character of you for them. It will just take some time. Just try to explain that you are not upset at all, you just simply don't agree with their opinion. People will eventually come to see that you are a nice person as well as someone who has an opinion.