This poem has a whole bunch of exclamation points in the first few lines. When you read the poem out loud, you can't help but feel some excitement, even some jubilation, at first. There are so many awesome things at the supermarket! I want to shout about the supermarket from the rooftops! I love food shopping! What peaches and what penumbras!
But after the first few lines, the poem takes on a sadder, slower tone. The poem is overcome by questions that are never answered, and let Shmoop tell you, a bunch of questions after a bunch of exclamations is one fast way to pop a bliss balloon. The exclamation points have all been replaced by question marks, and they weigh the poem down. We imagine the speaker whispering these questions to the imaginary Walt, knowing that he can never ever answer them. There's some real sadness in these lines; they contrast so deeply with the earlier, excited tone.
Furthermore, Ginsberg wanted you to read each of his lines in one long breath. It's not so easy—there are some pretty long lines in this poem—but hey, Ginsberg was always up for a challenge. In some ways, reading a Ginsberg poem out loud is a feat of endurance. If you feel exhausted after reading "Supermarket," out loud, well, you're not the only one. He probably did, too.