When we read this poem out loud, the first thing that strikes us is how neat the whole thing is. It’s so perfectly tied up. Every single line bounces along in perfect iambic pentameter, with no enjambment (lines running on into the next lines with no pause at the end of the line). Basically, the sound of this poem is perfect. Now on one hand that’s cool, because it’s really elegant poetry, but on the other hand there’s something weird about it, and it works well with the dominant themes of the poem.
Basically, the poem is almost too perfect. If the speaker were truly enraptured in love and completely obsessed with his beloved, we might suspect his words to come out a bit awkwardly as he tries to organize his intensely emotional thoughts into symbols. Instead, this poem is really crafted and sounds planned. In fact, the poem is so carefully put together that it gives us readers almost no leeway in how we choose to read it. Check out those last two lines: "So long as men can breathe, or eyes can see, / So long lives this, and this gives life to thee." Just try to read that in any way other than a really straightforward iambic pentameter. The speaker has completely locked us into his way of reading the poem. In sum, then, the sound helps us notice that this poet is more of a schemer than a lover.