This waltz has something the other five don’t: an introduction. So why did Sor do this? I suspect that context is the answer. After the brooding Valse No. 4, something brighter seemed called for, and Valse No. 5 certainly fills the bill. (It’s my favorite of the set.) But Sor was too subtle a composer to leap directly from melancholy to merriment. Rather, he eases us into it with the simplest of means: a repeated A, suggesting a dominant set up to the D major whirl to follow. Would Sor have omitted the introduction if this waltz had followed something different? Perhaps, perhaps not—we can never know for sure. But its place in the set goes a long way to explain its uniqueness.
Outside the music itself, context has the greatest influence on what a composer writes and how a performer interprets. Consider, for example, that in no other waltz do I add my own ornaments. Consider also that in no other piece do I linger on a note so conspicuously before continuing on my merry way. (A bit of the “great idea” that fell flat in Valse No. 3 found a home in No. 5.) Why? Because no other waltz in the set matches No. 5 for unbuttoned exuberance.