What's so great about reading? Well...everything. Being transported to another time and place, relating to characters, learning something new all make up the experience of reading.
See what others have said below.
Books are a uniquely portable magic. –Stephen King
Whenever you read a good book, somewhere in the world a door opens to allow in more light. –Vera Nazarian
Reading gives us someplace to go when we have to stay where we are. –Mason Cooley
There is no friend as loyal as a book. –Ernest Hemingway
Any book that helps a child to form a habit of reading, to make reading one of his deep and continuing needs, is good for him. –Maya Angelou
Great books help you understand, and they help you feel understood. –John Green
Between the pages of a book is a wonderful place to be. –Anonymous
Some books leave us free and some books make us free. –Ralph Waldo Emerson
A great book should leave you with many experiences, and slightly exhausted at the end. You live several lives while reading. –William Styron