Nat tells Kit that a person is loyal to the country he or she was born in because he does not feel loyal to the King of England.
Although the book takes place before the revolutionary war, there are definitely revolutionary stirrings already. Kit is a little surprised to learn that Nat has such strong passions about independence from England, just like her Uncle Matthew.
That would please Uncle Matthew anyway, Kit thought, bewildered and a little dismayed to glimpse under Nat’s nonchalant surface a flash of the same passion that made life in the Wood household so uncomfortable. Nat was a New Englander too … (Ch. 12, p. 130)
Nat’s point is that the colonists are more likely to feel loyal to the colony than England if they were born in America and not the old country. As time goes on, more and more colonists were born in American and loyal to America rather than England. This will turn the tide toward revolution as people because less and less willing to accept whatever England tells them and decide to rule themselves.
Kit is surprised that Nat’s hear beats with revolutionary fervor like Matthew’s. It never occurred to her not to be loyal to the king, because her grandfather taught her to. When he tells here there are “two sides to loyalty,” she begins to question her devotion to a King that has done nothing for the colonies but take from them.