The Latin term, "Memento Mori," can be translated as "remember you have to die." Acknowledging mortality can remind us of how precious life really is. Memento Mori was a popular theme in Victorian art and jewelry and has seen a revival through the steampunk movement of today.
Being a native of Tucson, Arizona, I probably see more cheerful art featuring fully dressed and active skeletons, or simple skulls, than most Americans. Much of this derives from the culture of our neighbors to the south, specifically the Mexican 'Day of the Dead' or 'El Día de Los Muertos' celebration.