The roots of Scottish nationalism run very deep and it is largely based on rhetoric and revisionism.
That they perceive that are somehow under the jackboot of the English who have been dragging them down all these years is so ingrained in the psyche of so many nationalists. Even on here it's like a constant refrain. I've read words today like 'tethered' and 'Scottish resources' and numerous references to 'the English' and 'Westminster' as well as some real venom from certain quarters about the UK in general, and it is that kind of ingrained victimisation and resentment is what fuels Scottish nationalism.
We live on a tiny island that would easily fit inside at least a dozen US states. It is about half the size of France with a similar sized population. We have an entwined and complex history that goes back two millennia if you look at the Brythonic and Goedelic Celts who populated these islands before the Romans came and subsequent settlement by Angles, Saxons, Jutes, Danes and finally Normans.
Despite the so called English oppression of Scotland it has a unique and mistakable cultural identity as do Wales and Ireland. No one has tried to suppress that other than in linguistic terms (and as a first language Welsh speaker and some who wrote a dissertation on the Welsh language and English attempts to erode it, I do know what I am talking about) but other than that Welsh, Scottish and Irish identity thrives.
I've been involved in 'nationalism' in a big way in my youth. Don't want to go into it to much but yes, I was. Young, patriotic, a Welsh speaker. If you want to google Meibion Glyndŵr - early 90s - you'll get the gist. Fortunately, I came to my senses.