Respect and Disagreement

I'd be very interested to know what other people's take on teenagers' disagreement with adults is. Especially those from other countries. My reason for asking is the following:

In Britain, in the 60s and 70s and 80s I was brought up to obey my elders - parents and teachers etc. Now I've lived in Germany for nearly 22 years and we have brought up our children to be polite, say 'please' and 'thank you' and not to argue rudely with us. If they disagree about something, an appeal to us is always legitimate - but a rude 'no' is unacceptable.

Having said that, I find that I come up time and time again against young teenagers here, who, if they think that I'm wrong, (and frankly, 99% of the time, I am afraid that they're wrong)  instead of simply saying, 'Excuse me, but are you sure?,' simply say 'No', with breathtaking equanimity - not aggressively, just completely sure that they are in the right. This comes across to me, with my English politeness, shyness and backwardness at coming forward, as incredibly arrogant.  It literally sometimes takes my breath away. I am sometimes so shocked, that I find it difficult to come up with a reply, because I don't expect such cool assurance. I suppose that I'm not that sure about myself, when it boils down to some issues.
I find it horribly difficult to answer such behaviour.

I don't think these children mean to be rude, but somewhere, some kind of respect switch is missing. What is it that their upbringing has lacked? Just yesterday I watched a You Tube video about a young man who had been brought up in Nigeria, in a rural Muslim family, and who had become a Christian after having some vivid dreams of Jesus. What struck me most of all was the beautiful way he respected his father. He never contradicted him, disagreed with him or accused him of being wrong; he appealed to him if he felt that he was in the wrong, but when his father commanded him to drink poison because he had left the religion of his forefathers, after appealing for mercy and then asking for time to pray for protection, he drank the stuff. Most Western children today would, I think, have displayed a somewhat different reaction.