Quote 1
It had occurred to Pecola some time ago that if her eyes...were different, that is to say, beautiful, she herself would be different." (1.3.18)
Pecola believes that physical appearance can alter one's psychological condition.
Quote 2
Thrown, in this way, into the binding conviction that only a miracle could relieve her, she would never know her beauty. She would only see what there was to see: the eyes of other people. (1.3.21)
Pecola lacks the self-esteem to not care what other people think.
Quote 3
She eats the candy, its sweetness is good. To eat the candy is somehow to eat the eyes, eat Mary Jane. Love Mary Jane. Be Mary Jane. (1.3.33)
Pecola's childlike thought patterns reveal themselves here: she believes that eating the eyes of a white girl could lead to becoming the white girl.