How Do You People Read?

When people are bringing up quotes in class, I do not know how they found them? When I read a book I only seem to pick up on the big picture and never seem to pick up on the themes in the book. For instance, I never picked up on the certain colors or the eye references while I was reading The Great Gatsby. The first time I picked up on it was in class when Mr O'Connor tells us about them. I do not know whether it is that I read too fast or am not careful enough? But either way I am not denying that they are there.