Well the people of Scotland were told the way to remain in the EU was to remain in the UK. Then 2 years later the people of Scotland indicated a desire to remain in the EU but were taken out due to the fundamental way in which the UK works as a whole. Many people in Scotland including the SNP see that as a democratic deficit, where fundamentally the people of Scotland don't get what they vote for (see also; Tory govt). On that basis, they continue to campaign for votes on the basis of a referendum, and they continue to get them. The other parties continue to be rejected.
So let's make it simpler.
On what basis should a referendum be granted?