One Week Friends

Hase Yuuki is interested in his unsociable classmate Fujimiya Kaori. Since he sits behind her in class, the only thing he ever gets to see is her back. One day, when they're alone in a hallway, he blurts out to her, "Would you become friends with me?". But her answer is, "I'm happy, but I can't."?! What could be the reason for this answer?