In the "Speaker" section, we talk about how we see the speaker of this poem as a kind of cheesy salesman pushing a product. That may be true, but this isn't necessarily a loud poem. A sales pitch can be slow and soft too. For some reason we hear this poem at a very low volume, just above a whisper.
You know when you are falling asleep, and a thought creeps into your head, something you've been worrying about maybe, turning over in the back of your mind for a few days? We hear this poem like that – not shouted from a stage, just a little rustle of a doubt, like the sound of wind through the door. The speaker isn't trying to beat the young man into submission, or force him to do anything. He's just trying to wear him down a little, to make him see things in a new way. All the speaker has to do is make the young man worry a bit, and he's done his job. Try it yourself. Say a line like: "Then being asked where all thy beauty lies" (line 5). You don't have to raise your voice to get your point across. The words are calm and smooth, and the sounds wash over you, instead of smacking you across the face.