Why are people's native languages so important to them?

I was having a conversation with friend who said that English is his native language and that his side of the family is from the UK. A friend of mine who is from the UK argued with him that he was lying and that there's no way he's really British. He went on and said that he can hear it in his voice when he's talking and that he doesn't have the same accent as the people from the UK. The argument went on with my British friend accusing him of lying about his nationality and telling him that he shouldn't claim to be something he's not. <br><br>I don't really understand why people are so proud of coming from a certain country or culture. It's not something you can control. It's not like you get to pick where you're born or choose your nationality. I've met people on this chamber from different countries who all have the same attitude when it comes to language. A lot of people from different countries say the same thing 'if you can't speak the language then you can't claim to be from this country'. What's the logic behind that? The United States for example is a nation of immigrants. A lot of Americans don't even speak English at home they speak some other language. Are they not American because they can't speak English? I just wonder why people are so proud of being from a certain country and why they care so much about where other people are from