I'm not sure Schubert is a composer you can explain but if you can, Rob Kapilow is the guy to do it. He presents a program for the Smithsonian here in D.C. entitled "What Makes It Great?" in which four times a year he takes apart a piece of music or a group of musical pieces. Last time (the first time) I heard him he was analyzing Cole Porter's songs. He showed how they are made up of amazing key transitions and all sorts of musical legerdemain.
Sunday he devoted his program to Schubert's Symphony No. 8 in Two Movements, well-known as the "Unfinished Symphony" to the world. He had the Peabody Orchestra with him to play each phrase, or for parts of the orchestra to play the phrase, and then after his hour's lecture, he led them in playing the entire piece. As an added lagniappe, at the very end of the show, after a question-and-answer session, he led them in what is the only part of a projected third scherzo movement Schubert finished--the first 20 bars. The student players were magnificent.
In that these 20 bars added little, I tend to go along with Kapilow in concluding that even if he didn't intend to conclude the symphony after the first two movements, it turned out pretty well anyway. He actually didn't finish a lot of things he started, and the man died at 31. This was written about six years earlier. The two movements might just be the most melodic symphony ever composed.
I don't think any other composer came up with the incredible range of melodies that Schubert did. My other favorite is the Quintet in C for string quarter and an added cello. It is not unfinished. In fact, it is long and yet you don't feel it ever lags or is padded. And I'd love to hear Kapilow expound on what Schubert intended with the strange couple of notes at the end. There were so many questions about the Unfinished that asking one about the Quintet would have meant changing the subject.
His usual inspirations were the mixture of love and pain, according to Kapilow. These also of course are the ideas behind the lieder for which Schubert is also renowned. The lieder, like Winterreise, make the sorrows of young Werther seem mild by comparison. And he was enthralled by Beethoven, too. Yet he composed in his own inimitable style, which does pay some tribute tio Beethoven but is in no way repetitive or even derivative. Just glorious in his own particular way.