Why do most Japanese classical pure music have very sad tunes?

I probably know what you mean by your question ...... Are you asking why the tunes of traditional Japanese tunes like Cherry Blossom make people sound very sad? As I understand it, it has to do with both subjective and objective factors, subjective factors are your personal feelings about the music and such, some people inherently think a tune is sad while others don't necessarily think so, which I don't have an answer for.

But the fact that a lot of people find this type of Japanese music sad suggests that there is an objective reason. I think the objective reason is that the "key" or "scale" used is very special. There are some peculiarities to this scale, which I don't really understand. If you look at it briefly, it's the equivalent of a 7-note scale without the re and so, with all the chromatic relationships between si do and mi fa preserved, and the incremental fourths between fa si, etc. These features have led to the writing of this scale in such a way that it has no re and so. These characteristics result in music written in this scale sounding rather cold-toned and even disturbing. The pentatonic scale, do re mi so la do, is much smoother than this one, with no semitone or augmentation relationships, so much of traditional Chinese music doesn't have the same feel as Japanese music. Most of the above is bullshit, but at the core it's the scale that causes it to sound weird and sad, and if you go one step further and ask why that scale feels wrong, it's a human physiological problem.

Finally, to correct the statement in your question. The statement "Japanese classical pure music" is incorrect.

First, the word "classical" is not used indiscriminately, it tends to be a Western cultural term. There are many such terms in the Western arts, such as "classical music", "classical literature", etc., and in the sciences, such as "classical economics", and in physics, "classical dynamics (classical dynamics)". In science, there are terms like "classical economics", "classical dynamics" in physics, and so on. Cultural concepts that developed purely in the East are not usually referred to by this term, such as Japanese music, which would be more appropriately called "Japanese folk music" or "Japanese traditional music". I know a lot of people generalize traditional things as "classical", whether Eastern or Western, but that's not strictly true.

Secondly, there is a lot more to "traditional Japanese music" than what you're talking about. Some of it uses the Chinese pentatonic scale, which doesn't sound that different from Chinese music, and a lot of other scales that are different, except for the fact that this one sounds sad to most people.

Thirdly, it's not necessarily "pure music". The well-known "Cherry Blossom" is not pure music, but a folk song.