A main contrast between the living couple and the ghostly couple is that the ghostly couple are back in the house searching for something while the living couple are simply going about their life.

We learn that the ghostly couple is seeking joy:

Wandering through the house, opening the windows, whispering not to wake us, the ghostly couple seek their joy.

It is clear they find it through memories of having lived happily in this house:

“Here we slept,” she says. And he adds, “Kisses without number.” “Waking in the morning—” “Silver between the trees—” “Upstairs—” “In the garden—” “When summer came—” “In winter snowtime—”

They also find it through realizing that the couple now living in the house is happy and in love. The ghostly couple sees the new couple and notes that there is "love upon their lips."

Finding the treasure they seek—a happy couple—the ghosts can feel that the house is "safe."

The ghostly couple differs from the living couple in that the ghosts derive happiness (or treasure) through the lives of another couple or through memories of the past. The living couple simply are happy on their own in the present moment.