This poem doesn't fall into any kind of established form or meter, but you can totally tell that Cummings wasn't just slapping random words on the page for fun. It might seem a little illogical at first, but the poem has a logic all its own. At first glance, the poem actually looks really tidy. Cummings doesn't do the crazy line spacing that you see in a lot of his stuff. Instead you've got five stanzas, each with four lines, making them...wait for the vocab word... quatrains.
Despite the stodgy look of the lines on the page, most of the poem lacks a formal rhyme scheme. Still, Cummings has some fun with rhyming words. In the first two stanzas he rhymes "enclose me" (1.3) with "unclose me" (5). Then he ties all this talk about closing together by comparing it to the closing of a "rose" (8). In the second stanza, he also mixes in some assonance by using "open" and "opens" (7). These words may not rhyme exactly, but the long O sound carries though it all, connecting this whole opening/closing motif he's got running through the poem.
Cummings brings the original rhyme back again in the third stanza, with "close me" (3.9). Then, in the final stanza, he pulls the most unexpected thing he could and gives it a regular rhyme scheme: ABAB (where the letter represents the end rhyme sound of each line). Check it out...
(i do not know what it is about you that closes A
and opens;only something in me understands B
the voice of your eyes is deeper than all roses) A
nobody,not even the rain,has such small hands B (17-20)
So, he basically morphs the "ose" rhyme from stanza 2 into the similar sounding "oses" rhyme in lines 17 and 19. Then, he ties it all together with this extended metaphor of the speaker's lover opening and closing him like a rose. The regular rhyme scheme created by rhyming "understands" (18) with "hands" (20) also gives a sense of resolution to the poem. It's like Cummings saying, "Hey, y'all. In case, you didn't notice, we're at the end of the poem, and this kind of sums up what I was trying to get at the whole time."