The brain devotes lots of effort to mapping where our loved ones are while they are alive, so that we can find them when we need them. And the brain often prefers habits and predictions over new information. But it struggles to learn new information that cannot be ignored, like the absence of our loved one.
Grief is a heart-wrenchingly painful problem for the brain to solve, and grieving necessitates learning to live in the world with the absence of someone you love deeply, who is ingrained in your understanding of the world. This means that for the brain, your loved one is simultaneously gone and also everlasting, and you are walking through two worlds at the same time. You are navigating your life despite the fact that they have been stolen from you, a premise that makes no sense, and that is both confusing and upsetting.