I didn't think it was possible for Mendelssohn to not be cheery - Variations Serieuses

While the writing here is very much in line with what is expected from Mendelssohn, the prevailing mood of this piece is decidedly not.  Sure, he has passionately written works - see his violin concerto, for instance.  But, to me, this piece is quite a bit different even from that.  I got a sort of intensity and passion in a way that is unexpected for him - something that goes beyond 'I write what I like to' or 'I'm trying to impact the listener's emotions.'  This work doesn't feel like it was written for the listener at all - which makes it no less enjoyable, but means that that enjoyment is secondary to the composer liking his own work.

I feel like Mendelssohn is always honest in his work - the exuberance in his happier works doesn't feel forced.  But then, neither does this work... he is able to capture something that is perhaps a bit outside his normal element, emotionally speaking, but composes convincingly.  Perhaps it is all of his other works being sincerely emotionally stable that keeps us entertained here, when Mendelssohn lets out the reins a little bit - still poised, still eloquent, he doesn't set out to let everything go as a Clara Schumann or a Robert Schumann when Clara's not around - but rather has us understand an emotion without having to see that he is absolutely in the throes of it.