While it is true that people require "air, food, water, clothing, and shelter,” in order to survive, we must also add "relationships" to this list because it is a rare person who is able to thrive in the absence of intimate relationships with other people, places, and things.
Grief is the process and emotions that we experience when our important relationships are significantly interrupted or (more frequently) ended, either through death, divorce, relocation, theft, destruction, or some similar process. A related term, “bereavement”, has different meanings for different people, but all meanings refer to the grieving process. While some view bereavement as a specific subtype of grief that occurs when a loved one (usually a spouse) dies, others think of the term as referring to the period of time during which grief is felt and losses are dealt with.
Grief starts when someone or something we care about is lost to us. We do not grieve for all lost relationships; instead, we grieve only for those that have become important to us over time. These can be relationships with people that we have strong connections to, such as family members, spouses, significant others, and friends; places we feel attached to, such as the house we grew up in or our hometown; or things that are important to us, such as love letters, a watch that a grandparent gave us, etc. We may have loved or hated that person, place, or thing, but we feel grief when they (or it) are gone.
There are two types of losses that we may grieve. The first is the actual loss of the person or thing in our lives. The second is the symbolic loss of the events that can no longer occur in the future because of that actual loss. For example, if a child is lost to parents, those parents lose not only their actual child, but also all the many events they expected to share with that child, including birthdays, graduations, wedding days, and other shared events large and small that make up the ongoing relationship with the lost child that is no longer possible because that child has died.
In many ways, we live our lives through our important relationships. Our relationships define us and who we are; they become intimately intertwined into our sense of self (or self-concept) and are thus a living part of us. It is terribly painful to lose one of these key relationships, because with the loss of such an important relationship, we also lose an important part of ourselves. For this reason, grief is not something that happens 'out there' in the world. Instead, it happens inside each grieving person's sense of self which is personally wounded and damaged by such losses. The work of grief is thus the personal work of healing and regrowing the sense of self.
Grief ends when we have gotten past the acute need for the lost other person or thing in our lives and are able to function normally without them. This doesn't mean that we stop feeling sad when we think about older losses; it only means that we are no longer significantly crippled by them.