Hi there Pere - as I'm here (popping in) I'll answer that question (that Ken did ask a few days ago).
The unification of Europe in the post-war (that's WW2) period has been fundamental to keeping the Peace in Europe - and comparing the past 50-60 years to any previous 50-60 years segment of time would agree that this has never been achieved before. A very positive thing and why amongst many reasons that I think the EU/EC/EEC has been a very good thing indeed.
It is now highly unlikely that Germany/France/Italy/Poland et al would even think about 'warring' against each other again. Job done.
And so, they don't need the UK to be a member any more to help maintain their Peace - they know how to do it now.
Borders - are one of the main things that people want to kill other people over. Also religion and other differences.
And so, whilst the EU likes to perceive itself as being 'borderless' it isn't at all because it is now trying to harden, and has been hardening the EU Border, also wishing to create an Army to defend it. Not only does this follow the same old, same old pattern towards War - it also begs the question what is the EU Border defending itself against?
NI/RoI Border - the nitty/griity question. Hardly anybody who lives either side wants to re-instate a Border, nor hardly anyone residing in the UK mainland, nor most sensible people do. Whilst most of the 'border' is created by rivers, the main idea of it is a man-made concept.
It currently works as well as it could in terms of Peace. The EU played a part in reaching GFA but it wasn't the chief negotiator, it's role was to provide the 'mechanism' through EU Membership, that allowed the UK and RoI (and othe interested parties) to agree to it. And primarily they all have and it's worked.
And so, similar to Continental Europe, learning by doing, living in Peace, so has NI/RoI - just not for so long.
If Peace was the ultimate Aim, then obviously no-one would want to feck about with the GFA. The only people that want to are the EU - who really should have agreed from the outset of Brexit that the RoI/NI no-Border situation as it is via GFA should be maintained.
But they haven't, and have plummeted in my estimation because of their stance on this. They're using it as a bargaining chip and maybe easy for them, over there in Brussels, so far removed, but shows to me how little regard the EU actually has in regards to Peace.