[单选题]阅读理解     From good reading we can derive pleasure, companionship, experience and instruction. A good book may absorb our attention so completely for the time being that we forget our surroundings and even our identity. Reading good books is one of the greatest pleasures in life. It increases our contentment when we are cheerful, and lessens our troubles when we are sad. Whatever may be our main purpose in reading, our contact with good books should never fail to give us enjoyment and satisfaction.      Why do we sometimes forget our surroundings and identity while reading（）