Grief is when you lose something you love. If love is defined as imagining all future realities with someone, grief would be losing something that you never imagined being apart from. Not because you expected your mother to always live for example, but because the positive parts of her will never be there again for you. So something that you had imagine all future realities including, is now gone. You have a void that will never be filled, depending on what you believe.
If you believe in the afterlife, you could hope to see them again. There would be at least one potential realty where you got to see them again. If you believe that they were going to the same afterlife as you, all potential outcomes involved the two of you together again, so grief becomes hope, becomes faith. And if you have faith that you will spend eternity with this person, you can still love them.