It kept playing on his mind the reason why she did not smile back when he gave her a small smile. He thought she was upset at him. He kept thinking of a smile for the next 3 days. Maybe, she did not have presence of mind or maybe she was pre-occupied with something else or maybe she was upset at some other thing. And also, if she did not smile back, why be upset ? You showed your humanity with your smile, why depend on a smile for the happiness of your heart ?
Sometimes the smallest things take the largest part of your heart.