Simply because the damages what the Japanese had caused to the Chinese were too hugh to compare what they had caused to the American. The Japanese raped, commitied genocide against civilians including children, destoryed our cities, plundering our national treasures, resources and gold, and worst they killed over 25 million Chinese people.
Moreover the American did get their revenge on the Japanese, they put the Japanese-Americans into the concentration camps, bombed the japs' cities and dropped two nuclear bombs and rule over them as a overlord. While the Chinese did none of that and even took care the Japanese children that left over in China.
As the result, the Japanese repect the American. On the other hand, many of the Japanese today still deny or downplay the Nanjing Massacre and comforted women, calling their invasion as "entering China"; their gov't insult us like using numbers as "731" or specific dates for their official announcements that associated with their past invasion. Not to mention many of their people visit and pay tribute to the Yasukuni Shrine that glorifies their past invasions which had caused alots of suffering in China. So this is always easy for the American to say let's forget and love the Japanese like a Mother Teresa, while we don't.
