As by the dead we love to sit
sit with the dead
Not literally sitting by corpses—this is about memory and absence. The dead become 'wondrous dear' precisely because they're gone.
grapple
Wrestling term. We're physically struggling to hold onto what's already lost, even though living people surround us.
broken mathematics
The math doesn't work anymore. Normal arithmetic can't calculate the value of what's gone—the equation is shattered.
penurious eyes
Penurious = poverty-stricken. Our eyes are poor, impoverished—they can only see what's here, not what's lost.