There is a kind of laughter that comes only to those who have plumbed the depths of despair, who have done their grief work, and who have learned to live a new way. Looking back clear of the resentment and bitterness, sometimes one can see the absurdity in the creatures we are, and love ourselves in our absurdity.
That laughter can only be had when one is no longer holding on to the past, no longer saying, "It should have been otherwise". It is very different than the laughter of embarrassment when someone too easily exposes their pain, or the laughter that tells us our pain is silly and unworthy of being taken seriously. It is the laughter of the deep experience of knowing ourselves fully and loving ourselves despite what we know.
There is no rushing grief. If a person is still angry/bargaining/despairing, you will not convince them to "look on the bright side". The pain must out, or we will carry it forever. But once it is out, if it is fully out, then joy is once again truly possible.