how is this tragedy related to the title of the book

ohn Steinbeck takes the title of this novel from the poem "To a Mouse [on turning her up in her nest with the plough": the poem was written by Scottish poet Robert Burns in 1785. In the poem, a farmer digs up a mouse nest and laments that all living creatures, including man, dies in the end. Mice, in particular, remember nothing of the emotional complex realities of life. Like the mice, Lennie does not have to remember any of the strife of the world.

While Lennie is much like a mouse, in which, he does not remember everything; the tragedy of this novel is because of Lennie changes and in doing so and having the brain of a 6-year-old he ends up hurting more than just himself he is a danger to not just himself but others as well. so in doing so his best friend George who acts like a parent to Lennie kills him as an act of kindness just as any parent would do to protect their kids.