The speaker holds back from someone they clearly adore, and the poem is about the reasons for that restraint. At first it sounds like a list of polite excuses: respect, admiration, even desire. But each one is introduced with "not because" or "not only because," so the speaker keeps knocking them down. They aren't staying away out of disinterest. They do crave it. The real reason only arrives near the end, after the word "Simply."
That reason is shame. The speaker feels they don't deserve to touch the other person, or even to walk and breathe the same air as them. Calling the loved one an "ethereal being" makes them seem almost unreachable, like something too delicate or too pure for ordinary contact. The title's image of clogging their own airsupply with their hands suggests the speaker is willing to suffocate themselves rather than get close. The mood is tender but sad, a mix of longing, self-punishment, and worship that leaves the speaker small and the loved one untouchable.