Classics endure for several reasons, but one of the big ones is that they are so real, so true that they expose human nature. Classics are fairly universal books that appeal to many different people over the course of generations.
If you want to understand what it means to be human, you can find the answer in a classic book. If there is a perspective you are having difficulty identifying with, there is probably a classic book that can lend you the perspective you desire. How would you handle war, famine, other hardships? If you prefer to find out without having the actual experience, a classic can help you have that vicarious experience. Classics allow us to view the broad range of human experience and perspectives. The more classics we read, the better we understand humanity.
Classics also grapple with the big questions of life. What is the point of it all? What does it mean to be a good person? How do my choices affect others? Classics have dealt with these questions and more. Classics don't always come up with solid right or wrong answers, sometimes they just explore the issues and leave the choices up to the reader. They do offer expanded perspective of these big questions, though.
If you have occasionally wanted to pick up a classic but avoided it for some reason, decide today to enrich your life by getting started. Classics are amazing!