I’ll take the OP at face value.
The reason why there would be a problem with a hard border is that many people in ‘Northern Ireland’ believe that Ireland as an island is a single country.
Ireland was invaded and occupied by Britain for centuries. A war was fought 100 years ago, after which Britain withdrew from most of the country. However, it held onto a corner of the country which had a high percentage of the descendants of British settlers (the Protestant/Unionist population).
To the non-settler population, a good analogy would be drawing an international border between Kent and Hampshire and telling the people of Kent that they were no longer English as England started after Kent. The people of Kent I’m sure would be outraged.
So the short answer is that there is tension because the border was artificially imposed by a foreign country and did not reflect the views of Irish people.
This situation is not unique. Palestine and India where the British also decided to play map-drawing have similar tensions.
There are interesting questions as to why the Protestant population didn’t mix with the native Irish. This is common with many British colonies e.g. America, India, Australia but not with other countries e.g. Spain/Portugal where there was much intermarriage with the ‘natives’