Grief is a Normal and Natural Process
There are a number of conditions that can make it harder for a person to successfully make it through the grief process. For example, sudden losses are harder to deal with than ones that have been anticipated. With anticipated losses, the knowledge that a loss will occur allows people to prepare, both by feeling grief before the fact of the loss and also by planning ways to minimize the negative impact of the loss when it does occur.
The loss of a spouse, lover, child, parent, or best friend is usually more deeply felt than the loss of more distant relations and friends. This is because such central relationships have long and deeply felt histories and an intensity of attachment that does not occur with more distant relationships. Central relationships are more deeply and significantly intertwined into the grieving person's sense of self, and thus leave a bigger hole in the grieving person's sense of self when they are lost.
The amount of support a grieving person can draw upon is critical to how successfully he or she will cope with grief. The more that friends, family and community are present and supportive, and the more that the grieving person is able to accept offered support, the better the outcome tends to be. Isolated people tend to have a harder time.
The "fairness" of the loss is also important. Losses that challenge a grieving person's ability to believe that the world is predictable are harder to manage. It is easier to accept the loss of an aged parent who has lived a full life than it is to accept the loss of a child. Death by disease tends to be easier to accept than death by a random, senseless accident.