Just have to keep a dialogue
Teach our bodies: haunt the cause
I was only trying to spell a loss

from Bon Iver – Calgary Lyrics on Rap Genius

Meaning

Some couples lose the romance or become bored as they grow old together. They…

have to keep a dialogue

…To survive the paces of marriage.

Teach our bodies: haunt the cause

Could mean they’re trying to remember what drew them to each other in the first place, what caused the attraction. Try to rekindle the spark.
I was only trying to spell a loss, Could mean one of them is afraid of the relationship disintegrating, or one of their deaths, and that’s why they’re bringing up all these worries.

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