The Japanese have words for sadness that are so subtle and complicated that the English translations don't do them justice.

Setsunai is usually translated as "sad," but it is better described as a feeling of sadness and loneliness so powerful that it feels as if your chest is constricted, as if you can't breathe; a sadness that is physical and tangible.
There is another word, too - yarusenai, which is grief or loneliness so strong that you can't get rid of it, you can't clear it away.