Most of Eastern Europe saw pro-soviet regimes installed and joined the Warsaw Pact; yet Finland, right on the USSR's border, a repeated thorn in their side during the war and occupying a strategically useful position on the Baltic, escaped this fate and maintained a precarious independence. Why did Stalin not push for the installation of a puppet regime here also? Was he prevented by the US/UK in some manner, or were other factors at play?
Not to discourage further answers, but you might be interested in this part of a larger answer I wrote.
The long and short is that in 1945, the USSR got pretty much everything it needed from Finland: a naval base, a legalized Finnish Communist Party participating in government, and Finland committing to neutrality and self-censorship around issues that impacted relations with the Soviets.
It's also worth noting that "Finlandization" (ie neutrality and disarmament along some similar model to Finland) was proposed by the Soviets for Germany, and ultimately the US and British preferred division of the country instead. So it's worth keeping in mind that in the postwar period, Finland wasn't necessarily considered a "loss" for the Soviets, but a dangerous precedent for expanding Soviet influence.