Just as all of the Warsaw Pact members were liberated from the Nazis in World War 2, so was Greece. Despite this, and their border continuity with Bulgaria, Greece was never a Soviet satellite state and was much more closely aligned with NATO. Why did this happen instead of Greece coming into the sphere of influence of the USSR?
If you're interested in a discussion of why and how the Soviets treated countries in Eastern Europe the way they did in the 1940s, you might want to see this answer I wrote.
I linked specifically to the conversation around Greece. The long and short is that by agreement with Britain (the so called "Percentages" or "Naughty" Agreement) in October 1944, Greece was to be predominantly in a British rather than Soviet sphere of influence.
At the time, the two major resistance movements in Greece were EDES (the National Republican Greek League) and the Greek Communist-led EAM-ELAS (National Liberation Front - Greek People's Liberation Army). By the time when German occupying forces were withdrawing from Greece in October 1944, these two groups were in a low-level civil war with each other. The British army, by agreement, entered the country that month and re-installed the Greek government in exile led by George Papandreou. EAM-ELAS participated, along with other resistance groups in a government of national unity, but when the British commander Ronald Scobie ordered the disarmament of ELAS, its government ministers resigned, and ELAS supporters staged demonstrations in Athens, which were fired upon by the police. The clashes escalated until the British army was itself engaged against ELAS, with a ceasefire agreement finally reached on January 11, whereby ELAS was disarmed. Its communist elements would reorganize and launch a second armed campaign in 1946, which depending on how you count things either was the start of the Greek Civil War proper, or the start of the third phase of an ongoing Civil War.
The Soviets largely respected the British militarily enforcing their sphere of influence, and most logistical and material support for the Greek Communists came from Yugoslavia, rather than the Soviets. The British continued to support the Greek government until March 1947, when the US began to provide substantial aid to the government. The Soviet-Yugoslavian split in 1948 weakened the Greek Communists as it meant the Yugoslavs withheld material support (fearing and preparing for a Soviet invasion) while Albania and Bulgaria (still in the Soviet camp) limited their support for the movement, and the war was effectively over by 1949.