Grief in a dream usually reflects a loss you're actively processing, whether that's a person, a relationship, or a version of your life that's ended. The dream can be an unusually direct space for that processing to continue, letting you feel and let go of something at a pace your waking mind hasn't allowed yet.
Mercury conflicts with those who are ruled by Jupiter, and Jupiter conflicts with those ruled by Mercury. - Picatrix (Andalusia, ~1000.AD)