You do mean 1949, because the NATO was founded on 4th April of 1949. And this was back then the USSR, and the new tensions between USSR and the West were still developing.
Aside that, some look at history and a map might be helpful, so let's have a look at a map of Europe with some markings done by me:
As it is evident, the border of Norway to Russia is small, while it is around 200 km most of this are mountains and rivers/seas, so area where an easy passage is possible is maybe around 40 km long, marked in black. This is something a good army can handle, and will not give major headaches to any country. Aside, as mentioned, the blocks were still building and heating up in 1949, so this is probably why the USSR didn't bother back then too much about it.
The border of Finland to Russia is a different beast, marked in yellow - around 1340 km long. It's also flat, and just mostly forest. That's something which is indeed hard to defend, also keep on mind Finland was founded in 1917, before it belonged to Russia. So the USSR definitely had issues with this border, which is why it fought two wars about it, the
Winter War in 1939 and lost. In 1941 there was the Continuation war, which the USSR also lost. Before these wars this border was always one full of tensions. So in the end after loosing two wars in a row against small Finland the USSR had a change of mind, and switched over to a "If you can't beat them, put them on a short leash" type of mentality, which lead to Finland becoming neutral and the signing of the Finno-Soviet-Treaty of 1948.
The Kola peninsula which I have also marked next to Finland is nowadays one of the main areas where Russia has its ICBM launch bases. Of course they came much later, but nowadays are a thing which affects security policies. There is only one major road/train line from Russia to this peninsula, not far away from the Finnish border, so Finland becoming a NATO member is for Russia a security nightmare, because sabotaging the infrastructure to Kola pensinsula in case of a conflict can be done easily by Finland, while it is almost impossible to defend that long border. Also Finland is one of the most militarised societies in Europe, if not the whole world.
Estonia and Latvia? Well, in 2004 Putin at least pretended to be still willing to lean more west ward. Back in 2004 three days after the Balticum joined Putin officially declared that he is quite happy about the improvements in the relationship of Russia with NATO. Also it's about 400 kms of border to Russia, and the vast population majority of Russia lives east of these two countries. So that's more of an annoyance probably to Russia right now.
Ukraine to Russia on land is 1974 km long, and much sparse populated. So much easier to invade Russia there.
Also now for some piece of history, this here was one of the initial planning of Nazi German on how to invade the USSR.
When looking about the first phase of attacks, 3 groups of the Wehrmacht were supposed to invade from nowadays Ukraine, one through the Baltic states and two through Belarus. And the Germans were not the first who enjoyed marching into the USSR/Russia through the Ukraine.
So whether we like it or not, nowadays Russia has its own opinion about the importance of that border for its safety, even more when taking history into account and given the fact that Ukraine is now independent as well.
Then there is also this famous quote by Brzezinski: "It cannot be stressed enough that without Ukraine, Russia ceases to be an empire, but with Ukraine suborned and then subordinated, Russia automatically becomes an empire."
So this puts Ukraine into a very special, delicate place and position. And that's the reason why Russia also has its own mind about Ukraine being in NATO. The tough question is how to deal with it.
www.bbc.co.uk
