Japanese people in old days never used sweet words (current Japanese either) such as "I love you." and so on. So, when Sosuke Natsume translated "I love you" to Japanese, he recommended that it be said that "月がきれいですね" - Tsuki ga kirei desune (means that the moon tonight is beautiful, isn't it?)
In his opinion, when you are with a women you love and said "the moon is beautiful, isn't it?" to her, it is as valuable as saying "I love you."