Grief is a manifestation of the experience of grief over loss. There are five stages of grief. Grief refers to the fourth, depression. This is the period when a person is already able to grieve. At this stage, you can lose interest with yourself and other significant people. Vital meanings are lost. It is very important to stay close to the grieving person. Even if there is an impression that he is reveling in loss. This is partly true, and this is a good signal that there is an opportunity to move to the stage of accepting the loss. Survive the grief. Then the meanings and desires gradually return. It's important to experience and deal with all stages of mourning, from denial to acceptance. And grief can only be experienced in contact with other people.
It all depends on how close the person was and how you treated him. There are two options: either you grieve because YOU will never be able to see him or communicate with him again, or you grieve that he died, and the whole world died with him, his own world, for example, a person dies of cancer, and the next day it turns out that there is a cure for it in every refrigerator, that it is almost free, etc. So everything is dualistic.