Those of us who teach English speak of literature as providing us with windows and mirrors, and these are the ways we connect themes to real life. Let's talk about each of them.
What is the purpose of a window? It is to look outward, to see the world outside of our own existence. When we read literature, it can be like looking out a window, which helps us to understand how our lives might be the same as the characters that we read about and also how our lives might be different. A theme in a book, which is a message the writer would like you to take away after having read the book, is an important part of that view through the window, one that can easily connect to real life. For example, The Giver, by Lois Lowry, has as its main theme the struggle between safety and freedom. In the book, the protagonist, Jonas, has a choice between the two and opts for freedom. This can connect to real life easily, since every society must grapple with that choice, to allow its people great freedom or to try to keep them perfectly safe. As you become a participant in the democratic process, your vote might very well help to decide that issue. In The Kite Runner, by Khalid Hosseini, an Afghani novel, two themes are betrayal and redemption. Amir betrays his friend Hassan and then redeems himself by saving Hassan's son. How does this relate to the real world? It allows us to understand that people all over the world grapple with these problems, how to be a good friend, how to make amends when not, and this affords us some understanding of another culture and empathy for others. Good books act as windows that allow us to look out into the world and make connections with our own worlds that are thought-provoking and often helpful.
A mirror allows us to look inward, to examine ourselves. And literature serves that purpose, too. A universal theme, which is one that applies to all of us, helps us to view ourselves more critically, to measure what we are and do against the character and behavior of the characters in the story. One example that comes to mind is Shirley Jackson's "The Lottery." In this story, the characters blindly follow a horrifying tradition. One theme of the story is that people should not always blindly follow tradition. This theme is meant to help the reader examine his or her following of tradition and ask if it is a good tradition or a bad tradition, and whether or not the reader should continue to follow it. Another example that comes to mind is Sue Monk Kidd's The Secret Life of Bees. This story, which takes place in the civil rights era of the sixties, shows engrained racism in the American south. One of its themes is the painful struggle for racial equality. An astute reader is likely to measure his or her behavior and attitudes, to question whether or not behavior and attitudes are congruent with racial equality. These are all stories that will cause a reader to look in that mirror to see how the themes are borne out in the reader's own life.
These books and many others continue to be read because they do have themes that allow the reader to make a connection with real life, whether it be by looking outward or by looking inward.