How is the theme of death presented in the poem?

The theme of death is a key theme in Tennyson's poem Tears, Idle Tears.
The focus on the "days that are no more" goes hand in hand with the haunting realization that death affects us and everyone we know as we live our lives. Our happiest moments always have a sadder side to them because they are temporary. This caducity brings us to the realization that these moments cannot last, not only because life goes on but also because at a certain point we inevitably encounter death. 
However, it is exactly this that gives value to our memories and happiest moments. The true beauty of life is appreciated thanks to its juxtaposition to sadness and death, and it is our realization of the inescapability of death which pushes us to appreciate life and truly strive to create beautiful things. 
By being temporary, life is even more precious and beautiful; if this were not the case, life would simply be taken for granted and not appreciated. Life is both beautiful and painful at the same time, and for this reason we must try to live it to the fullest.