What forms the nationality?

OK, as you probably see on the left, I'm new here, so I'm starting off with a question. Lets hear/see some personal opinions from the pros.

What forms the nationality of a person? Is it the place where they were born, is it the place they grew up in? Or maybe it's the influence they've had from the people/family members around them? Or is it their personal choice as they come of a proper age to have an opinion on the matter? What makes a person British, German, Russian, Austrian, Polish, Bulgarian, American, Canadian etc. ?