The speaker of "Lullaby" spends a whole lot of time thinking about death. In the same sentence, he calls his beloved both "living" and "mortal." While he's alive now, he won't be alive forever. The speaker really hammers this idea in. Then, in the climax of the poem, the speaker announces that we should "find the mortal world enough" – that we should be satisfied with the here and now, and not look to God for answers. We should accept the mortality of all life and rejoice in the meaning that death provides. Sure, it's not exactly the cheeriest idea for a love poem. But there's something lovely about the speaker's realistic outlook on life and death.
"Lullaby" is the most depressing love poem ever. Auden can't stop thinking about death, and this makes the poem a huge bummer to read.
"Lullaby" is an incredibly realistic love poem. Auden's focus on death makes love seem all the more poignant and beautiful.