(Psychological / emotional perspective)
Psychologically we all seek within ourselves complete harmony in partnership.
This is a security that allows us to express parts of our personality which are not normally accessible otherwise. Dreaming of being a widow or widower when we are not suggests that we have lost part of ourselves.
A husband represents the masculine principle, the drives and ambition, and a wife the intuitive sensitive principle, so to have lost either is to have lost an essential part of the mystical union that everyone requires in order to become spiritually complete.