I love Sibelius. A great and sombre waltz!
I think (though this is going out on a big limb), Sibelius' realisation of how effective very basic musical ideas can be emotionally, is one of the things which lead to the great melancholia of his later life. Take the 5th symphony (I'm a knuckle head, I forget whether it is 2nd or 3rd movement). The movement where there is an extremely complicated build up into what is a rather basic climax, one of the most rapturous imaginable but yet pretty simple in most ways. It is the same with the first movement, the opening theme coming back after great complications all breath-taking and all. I think maybe Sibelius negatively came to the conculsion it was a cheap trick, the whole lot of it, and surely not just his music either. I'm just using this as a possible factor, not that I'd really know, simply a hypothesis.