Christopher Gibbs, in his program note on the symphony, wrote:
The appearances of the Tell Overture in the first movement (Allegretto) are so striking that one inevitably asks why Shostakovich inserted such a well-known piece. The composer himself divulged little, except that the first movement “describes childhood— just a toyshop, with a cloudless sky above.” Indeed, there is a largely playful tone to the movement, although the conductor Kurt Sanderling recalls sitting with the composer at the Berlin premiere and remarking that, unlike most of the audience, he found the first movement tragic. He reports Shostakovich replied, “You are not wrong. It is tragic, marionette-like: We are all marionettes.” [cite]
It’s virtually impossible, with Shostakovich, to avoid looking for clues in his music. The speculation about quotations from other works in Symphony No. 15 is likely to be without end. But perhaps, from time to time, we can simply listen. Bernard Haitink, when asked about the reason for the William Tell quote, said
Because the Rossini fits perfectly. Shostakovich himself said he didn’t know why, just that the quotations had to be there. Why is it that we need answers and explanations from Shostakovich all the time? This is great music and his reasons lie in the music. [cite]
Prufrock's Dilemma 
Susan, thank you for this post and for continuing to blog about music and poetry through this isolating time. Aside from the occasional live streamed home concerts, music has not been the refuge for me that your blog provides. The tight structure and clean prose of your writing is just as much a space for breathing as the intellectual content it contains. Many thanks, again, and I am sharing your post. Lowell
Lowell! It’s so nice to hear from you. I’m so pleased these small posts of mine may provide some refuge, and how nice of you to share this post. May you and yours be well and stay safe.
Sue, whatever has finally persuaded you to return to your blog, to your beloved Shos, and entice us all along with you, I am grateful. After such a long spell of quiet, it’s wonderful to find announcements of new posts from you. I’m as happy as a clam! — Warm regards, Elizabeth
Elizabeth: Great to hear from you, as always. Well, I’m not REALLY back, but from time to time, I am reminded of or have recommended to me a piece of music and don’t want to lose track of it, so the blog is a good repository. FYI, most recently, I’ve been taking a class on the symphony from our mutual friend David Nice. It was excellent, you won’t be surprised to hear. (Just ended this past week.) He is planning another, on Russian music, so I’m looking forward to getting news of that.