In a poem called "Home Burial," death has to play a big part. It just has to. But in this poem, it's pretty much the elephant in the room. It colors every line of dialogue in this couple's conversation, but they hardly ever mention the word. It's almost as if, for this couple, using the word "dead," or talking directly about the death of their child, makes his passing all the more real, and their grief more painful. But death is real, and it's driving a wedge between these two.
Her child's death has caused this woman to be consumed by thoughts of mortality, which means she can't actually live her life to the fullest.
The man's attitude toward death is unhealthy, because he won't talk about it directly.