You may want to check out John Dower's Embracing Defeat. Also I feel like I've seen this question on this sub before.

Two things I would point to. First, Japan was in shambles following World War II and the United States helped it recover. There was widespread poverty and famine as so many resources had been used up or destroyed during the war and Japan no longer had colonial holdings to support itself. Much of its infrastructure had also been destroyed. Many Japanese were scattered or displaced among Japan's colonial holdings, meaning separated families. And moreover (and this is Dower's thesis), the loss struck a huge psychic blow to the Japanese, creating an air of defeatism.

The Americans, though they were the enemy during the war, led reconstruction efforts in Japan and were essential in revitalizing the Japanese economy. General MacArthur, who lead the Supreme Command of Allied Powers (SCAP), had a cult of personality in Japan; the Japanese for the most part adored MacArthur. SCAP did not prosecute the Emperor and a compromise was reached regarding the Emperor's inviolability. Essentially, the United States led a well-received reconstruction effort that helped Japan rise from the ashes.

Second, Japan and the United States grew close because of Cold War politics. Japan, threatened by communism from China and the USSR, needed a regional ally. The United States became that ally. The two countries fought together during the Korean War (which incidentally also did wonders for the Japanese economy, as Japanese manufacturing was essential to the war effort in Korea). Cold War politics also explains why the United States spent so much time and resources in rebuilding Japan.