To Kill a Mockingbird has become a timeless classic for a variety of reasons:
1. POINT OF VIEW: This is told through the eyes of a child seeing the atrocities adults inflict on each other. Because of Scout's innocence, readers experience the circumstances as if they are children. We always side with a main character. The difference between the reading audience and Scout though is the fact that we know better. This point of view makes us pay attention to the terrible things adults do to each other in our current societies.
2. THE THEMES OF INNOCENCE OR INJUSTICE: No one likes to see others treated terribly. Harper Lee uses the metaphor of the mockingbird and several innocent characters who do not deserve the treatment they receive: Boo, Tom, and even Mayella. These treatments vary too from very direct like Tom's death, to indirect like neighborhood gossip.
3. PERSPECTIVE: Learning to walk in someone else's shoes or imagine life from someone else's circumstances is brought up several times. This idea comes full circle by the end when Scout is standing on Boo Radley's porch.