Everyone is entitled to their opinion...except when it is different from our own. There's a difference between respectfully disagreeing with someone but then telling them that they're wrong without listening to their perspective. We like to say that everyone is accredited to their opinion. The sad reality is, we are likely to be right yet we never want our opinions to be wrong. We want to believe that our way is the best and the only correct way.
This issue allows us to be close-minded and naive which exists in every day life. Most of the time it is overlooked and people don't realize that they are dismissing someones belief. We say that the other person's argument is invalid because of their interests, knowledge, or personal background. You may tell them that they're being rude or mean, but they will probably just tell you that they have a "right" to their opinion. Well, yes they do, but that doesn't mean they have the right to be a jerk.
We need to learn how to have respectful discussions with people that we don't necessarily agree with. If we listen, then we might understand their perspective. This really comes down to the Golden Rule: Treat others how you want to be treated. You wouldn't want someone to dismiss what you have to say. So why do it to someone else? By dismissing their opinion, you are dismissing a basic human right. You eventually might find that you agree with them on certain aspects of whatever topic you are discussing. Even if not, then you have had an intelligent conversation with a person who does not agree with you.
I might be repeating myself, but I don't think it can be said enough: be respectful and maybe you'll find that the girl you think disagrees with you on everything actually has some good ideas or the reason guy from the opposing political party believes what he does.