Having presided over a number of funerals, I have observed that mourning results from the separation and sense of loss that we all have at parting from someone we love. But those who live without hope in the resurrection and in the redeeming power of the atonement of Jesus Christ suffer a different kind of loss. With faith it's possible to feel sad and mourn for the loss of our loved ones and at the same time look forward with hope. The Lord spoke to Joseph Smith and gave this direction (D&C 42):
45 Thou shalt alive together in blove, insomuch that thou shalt cweep for the loss of them that die, and more especially for those that have not dhope of a glorious resurrection.