I get all that, paddyann - my point is not that people don't feel that way, as it's clear that many do - but to ask why it happens.
To go back to the 9th century, the king of the Scots would be fighting with the king of Northumbria and both of them would fight with the other kingdom on the borders, that took up parts of what are now Scotland and England - Benicia?
Who knows why? Territorialism, probably. There would be gains and losses over the years, and some ancient families will have lost and gained land and titles as a result. The vast majority of people would have had enough to do staying alive and feeding themselves without being bothered about who owned which bits of the land that they paid rent on. One king would be much like another, and probably none of them would be benevolent rulers - they were all thugs, really (on all sides).
Also, people have always come and gone over the various borders, which haven't been static anyway, so very many people are 'pure' English/Scots/Welsh/Irish anyway. I am technically English, but have Scots and Irish ancestors that I know of, (and maybe all sorts of others If I could go back to the 9th century) and many, if not most people on this tiny island will be similarly 'mixed'.
The bit that I find difficult to understand is that all these years later, people still bear allegiance to the old boundaries, and I wonder why.