Why did truman believe greece needed american aid in 1947?

To stop Communists from gaining control of Greece. 
Since the end of WW2 the British had been supporting the government of Greece, and Turkey, with economic and military aid but in February, 1947 the British officially announced to the U.S. that they could longer support Greece, or Turkey... the responsibility was passed to and accepted by the U.S. 

In March, 1947 Truman stated "It must be the policy of the United States to support free peoples who are resisting attempted subjugation by armed minorities or by outside pressures." It was known as the 'Truman Doctrine' and it became U.S. foreign policy for many years, it led to America's involvement in fighting Communism during the Korean War and Vietnam War.

Truman believed Greece needed American Aid to stop Communists from gaining control of Greece.