Awareness of the inevitability of death need not cause sadness. Rather we can use it to destroy the common worries about inconsequential aspects of life. Many worries are over matters that have no lasting value. When you overcome worry, your mind will be free to think of your ultimate goals in life.
The physical loss is not sufficient for mourning. Purely on a physical level what would a person gain if he lived many more years? What is the ultimate gain in devouring hundreds more chickens and thousands more loaves of bread? What is the overall difference if the deceased left all this to others? The Torah obligates us to mourn to emphasize the loss of the true value of life; which is the spiritual elevation a person could have gained if he were still alive. The Almighty placed him on this earth for this purpose. The person’s death should remind the mourners to fill their lives with the spiritual growth that they are capable of.