Relationships.

Siblings: England, Scotland, Wales, Northern-Ireland and Ireland are his brothers. No i do not see all the states personified, but i guess everyone is different with their own ideas about Hetalia, the reason why Cornwall is a personified county is because he was always seen as a separate country and therefore he was 'born'.

Cousins: The overseas colonies, Australia, Bermuda, etc etc etc. Although Cornwall is not very familiar with many of them due to his preference of being at home in his county.

Friends: Brittany, France and Italy have always been nations he got along with, they had always seen him as his own country even when England did not.