Juxtaposition, according to its dictionary definition, means placing two things next to or adjacent to each other. In poetry, this could be done for several reasons. These reasons can also provide the clues as to where the important juxtapositions lie.
Some of the reasons why a poet might use juxtaposition include comparison, contrast, rhetorical effect, or suspense.
In the poem by Page, the words “either” and “neither” could be juxtaposed for several reasons. The first, and most obvious, is their lexical closeness. Only a single letter separates them. This could have some effect upon the meaning of the poem, which appears to be filled with contrasts.
These words might, for example, contain the central meaning that, while they contrast in meaning, they appear to be quite close in form. As such, the very nature of the words forms a juxtaposition.