[Smaug123]

For what it's worth, you asserted above that you are not rude, but telling a stranger they sound like a sociopath is generally considered rude. It may genuinely be that you have transcended social norms, but the rest of us haven't, and I'm afraid you're dealing with the rest of us!

[NewOrderNow]

Why is this considered rude instead of stating an opinion?

[krageon]

wow you sound like a serial killer

Perhaps that serves to illustrate why it is rude. If that doesn't help, then just remember that it is. Good manners don't always need to make sense to you, sometimes they only make sense to other people.

[sdwa]

They aren't mutually exclusive—an expressed opinion can also be rude.