The speaker is a bit obsessed with the ideas of death and the passing of time. This sweet little love poem is actually filled with morbid images and thoughts. Does love make the speaker think of death? Does death make him think of love? Probably; the two seem really intertwined in this poem. It's like he can't look at his beloved without being reminded that they're both going to die someday.
Lines 3-6: The speaker starts off the poem incredibly sweetly and then transitions right to death, saying that all children will die some day. Nice way to set the tone, Auden.
Line 9: When describing his sleeping lover, the speaker calls him "mortal, guilty." Sure, he follows that up with "entirely beautiful," but did we really need the "mortal"? Actually, yes, we did. The speaker loves the beloved in spite of the fact that he will die. All romance for the speaker is doomed romance, because no one, and no relationship, can actually last forever. That's what makes it all the more special for the speaker.
Lines 21-23: The speaker invokes two images of time passing: a clock striking midnight and a tolling bell. He's not just talking about time and death now; he's bringing images of these abstract concepts into the poem to make them all the more real.
Line 27: Yet another symbol: tarot cards, which predict the future. So what's in the future for us all? Death.
Line 31: Now we've got another list of things that die: "beauty, midnight, vision." This is definitely ruining the mood.
Line 36: This is really the heart of the poem. The speaker tells his beloved to "find the mortal world enough." It's like he's telling him to be okay with death and to look for fulfillment in this world (not through God). Auden almost embraces death here, and realizes that it's what makes life beautiful.