In answer to the thread title, yes.
Because we had dreams that we did have it, or would have it, if only xyz happens.
We're not mourning the loss, we're mourning the loss of the dream, and the loss of ourselves in pursuit of the dream-that-can't-be.
It is hard to accept reality instead of a dream-that-can't-be, and the more unreasonable the dream, the harder to replace it with the reality of the situation.
I am working on replacing my dreams-that-can't-be with good, healthy dreams that should be. (Not the "should-have-beens" but the "I-want-to-accomplish-this" kind.) It is helping... But it is still painful.