When most people think of grief, they think of the loss of a sibling, a parent, a child, a friend, an animal companion. These are truly a cause for mourning, whether that person or animal has lived a full, long life or not. Some people feel guilty missing and crying over the loss of someone whose time has come, but the loss is still real and still painful. It can be worse still when the loss is unexpected and/or premature. It's hard to say whether seeing an impending loss coming or a experiencing sudden loss is worse. Either way, it hurts. There's no real point in considering gradations of pain and whose loss is harder when it all hurts and can be life-changing and all-consuming. There are losses people don't consider when the words "grief" and "mourning" come up. However, I have seen people in mourning just the same.
There are human losses that don't involve death. This is considered ambiguous grief. I've seen people hesitate to mourn when they know they may never see their child/parent/aunt/uncle/grandpa/etc. again in this life. There are people who can't respond when you speak to them, like those in a coma. There are those who are no longer the person we grew to love, like an alzheimer's patient. Their loved ones sometimes seem to feel like like they haven't earned the right to mourn. However, I've heard experts in the field validate this kind of mourning. You have still lost the one you love. And it still hurts. It's okay to grieve, however you feel you need to grieve.
There are losses of expectations and familiar places like watching future dreams figuratively go up in flames or seeing your home or work literally go up in flames. A mother who dreamed of sending her child to college, who then finds out he has severe disabilities, or a father whose child, so full of potential, makes a mistake that costs her a future still faces pain. It's still a loss. It can be hard, and can hurt.
This isn't to say that all these losses are somehow equal, that mourning is mourning, and it all looks the same. If I lose my job, there's no way I will be launched into deep mourning as I would be with the loss of a parent or child. However, that doesn't mean all of these forms of grief aren't valid. They are, and those suffering from loss deserve respect and understanding. Nor does this mean that all people who lose a child or all people who lose a job go through all the same things or should be expected to grieve the same. Grief is hard. Loss is painful. We often don't know what others are going through or how much they have lost. For this, we'd all be happier and have a better chance at healing if we treat each other with respect and understanding, so when we have our loss and pain, others will be there for us.