When the relationship was difficult
This one is almost never talked about, and it is very common.
If the person who died had hurt you, or if things were unfinished, or if you were relieved as well as devastated, then you are grieving two things at once. The person, and the version of them you were still hoping for.
Relief and grief arrive together in these cases, and the relief brings guilt with it, which then makes the grief harder to feel cleanly. That is not a defect in you. It is what happens when a complicated relationship ends in the one way that cannot be repaired.
This course does not go back through that relationship with you and it does not ask you to write anything to the person. If it is the largest part of what you are carrying, that is a good reason to see somebody.