In the "The Great Gatsby", Notice how many times the author uses the words hope or dream. Why does he do this?

The theme of the book is hope and dreaming for something that cannot be attained. When Gatsby sees the green light flashing over the river where Daisy lives, he is reaching out to his past, his hopes, his dreams. Daisy is the embodiment of Gatsby's hopes and dreams. Unfortunately, Nick learns (though Gatsby doesn't until too late) that you can't actually recapture the past, only simulate it in superficial, self-destructive ways.