Because literature makes your "everyday life" so much larger! Not only does it allow you to explore worlds that don't exist or that are in times long past (or times to come), but it lets you walk in someone else's shoes. It develops in you the concept of feeling for a person whose pain is a different sort than yours. It lets you peek into troubles you may not personally experience and then care about them. Even when the book is light and happy, you're still spending time with someone else and caring enough about them to "listen" to their personal story.
In short, I believe literature teaches us about people and the more we consider those around us, the more open our lives are.