> It's largely because Finland has never felt a need to be in NATO.
Well, at least Finland never said it felt a need to be in NATO... But there are more reasons not to say you feel something than not actually feeling it.
> During the cold war Finland was firmly Soviet aligned despite all the claims of neutrality,
At least it certainly behaved like it was "firmly Soviet aligned"... But there are more reasons to behave some particular way than one's own volition or conviction.
> the Finnish president, Urho Kekkonen, was de facto a dictator
Whoa! Yeah, sure, that's one way of putting it... But certainly the most drastic possible one. There are other equally valid and significantly less drastic words for him.
> who stayed in power through Kremlin's support
Well, or through the Finnish people's and politicians' trust that he was the best suited, through personal connections and enjoying the Kremlin's trust in him personally, at navigating the tricky relationship with the big neighbour. OK, that's kind of "through [the][1] Kremlin's support", but only indirectly and mainly as modulated through the Finns' own confidence in him. Above all, when the time came to retire him, they did (although given the slight farce of those last few years, yes, it could have come some years earlier).
> (although he wasn't really bad for Finland)
> and Finland had a defence guarantee from the Soviet Union in the form of the YYK-agreement.
Well, it's hard to claim that was much of a useful "guarantee", given who the only plausible aggressor one could have had to defend against was... It's not like Sweden or Norway were ever very likely to attack, and Estonia didn't exist as an independent nation a the time. The only nation that had ever attacked Finland was the Soviet Union. So that was pretty much a null clause of the agreement.
> After the Cold War Russia was expected to become a western liberal democracy and as such not a threat, and even after it became clear that it wouldn't become one people still understood it as a "known threat" and a bully which would act tough and try to intimidate but who would never actually hit you.
Hmmnyeah, OK, that was pretty much the consensus -- at least as far as what was possible to gather from public political debate, so the official "consensus" -- but there certainly were also other currents among the population at large, from at least as early as the fall of the Soviet Union.
> The largest reason for why joining NATO got a surge in polls is that most people weren't expecting the Invasion of Ukraine (I myself believed that they would've limited themselves to just Donbass and Luhansk).
The largest reason for it surging now, definitely. But...
> I expect, although I may be wrong, that once people calm down support for joining NATO will fall,
...but that doesn't necessarily mean it'll fall back to earlier levels once this war fades from the front pages and the top of the TV evening news: It may have raised the ground level permanently, by convincing at least some of the erstwhile NATO-naysayers that they had been too optimistic about Russia.[2] As you say you were. Would you now say the invasion of Ukraine has changed your personal assessment of risk from Russia permanently, or will you in the future think it's only as dangerous as you thought before?
> although the FUD spread by NATO supporters about how we would need to fight Russia alone if were not in NATO despite being covered by EU's article 42.7 may keep it high.
Sigh... It's not "FUD"; it's the folks yelling "EU 42.7 is just as good as NATO 5!" who are spreading a falsehood. Dunno if they actually believe it or are consciously spreading deceitful anti-NATO propaganda; probably some of this, some of that, and possibly even some in the middle who aren't even sure themselves.
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[1]: Ootsä suomalainen? Teilläkin on slaavien kanssa tuo article-ongelma yhteistä. No juu, muu tekstisi alla ainakin viittaa melko vahvasti siihen suuntaan että olisit. Mutta olitko elossa Kekkosen aikana? (Itse olin, mutten Suomessa.)
[2]: It's hard to tell, since I personally never was. I can't think of any old dateable writings to prove it, but I've been expecting the other shoe to drop on Ukraine since at least the invasion of Crimea.