Some boxes are complicated, others are filled with love.
Anna and Tim are stuck in a difficult Beziehungskiste - literally, "relationship box." Beziehungskiste is the colloquial term for a relationship that isn't going particularly well. "What a complicated Beziehungskiste!" a friend of Mara's might say, or "You've gotta get out of this Beziehungskiste!"
Some people really do have a relationship box - in the literal sense. They keep their memories of a lost sweetheart safe in a box: love letters, cinema tickets from the first date, jewelry and other mementoes. That way they can remember the good times, even if they found themselves at times in a trying Beziehungskiste.