Fragility of Life is a central theme because Oskar's Dad and Anna were both lost unexpectedly and with enormous effect on Oskar and Oskar's Grandfather.
The fragility of life is a reason for many things in the book, particularly Oskar's Grandfather's reluctance to love Oskar's Grandmother, and Oskar's efforts to remain distant from his own mother. Both these characters are hesitant to get close to anything, for fear of losing it and suffering.