They may not be angry or unhappy, just joyless. Look at it as a scale. Anger or bitterness is on the far left. A stable, sedate personality is in the middle, and joyfulness is on the far right. Granted, children do far better with deadpan parents who have no joy than they do with angry or bitter parents, but they do best when both parents are known for their joy.
Bitterness is a plant with a disease.
Joyless mediocrity is a plant without disease growing in average to poor soil. Joyfulness is a plant rooted in well-balanced soil with the right combination of rain and sunshine. Parenting, like courtship, must be properly seasoned with joy. Parenting without joy is not only tasteless, it is tiresome. Parenting without joy is like music without rhythm or flowers without color.