One of the hardest parts of losing someone close is the loss of role; you have to redefine yourself again, and that is difficult and uncomfortable. You're no longer a spouse, or a parent, or someone's child or sibling. The connection to role plays a big part in our feelings of safety in life. When we feel that we know who we are, how we fit in with others, how they mesh with us, we feel comfortable enough to explore other things rather than identity.
Nobody even gives you a word for it, unless you lose a spouse; then you are "widowed", and the implications are dark, stark and full of alone. Even the merry widow is responding to her loss with rebellion. If you are a child, and you lose a parent, you are an "orphan". But no matter how old you are, when you lose them, you still feel like one. That doesn't pass just because you are an adult. What about losing a sibling, a person you grew up with, for good or ill? Or a child, heaven forbid, what do they call you then? Your loss has no parameter, even if it is still such a role loss that your mind can't wrap itself around it.
In some ways, in your most intimate relationships, you take on parts of the roles that your loved one is missing, and they do that for you also. I know some people would call that co-dependence, and it is, but not in a pathological way. While it's important to be who you are, and to have boundaries and your own aspirations and goals, the human connection, recognition and nurturing of that spark that holds us together, is more important than any piece of paper, any material thing, any job or recognition the world can give you. Defining who you are within a relationship, feeling secure to expand and softly intertwine so that you support each other through life, is, to me, one of the most beautiful parts of being alive.