When you first confessed to Obito, you were only 12 and he was 15. The response you got was, "Oh, but you're too young for me, sorry."
After that, you didn't look for him again. You are currently 15 and Obito is 18. The holidays had passed, and it was the first day of school in a long time. You were walking down the hallway looking for your friends when you saw a group of boys laughing and chatting. You just walked past, ignoring them.
Inside that group was Obito, who couldn't help but follow you with his eyes. Was she the same girl he had rejected 3 years ago? You were a beautiful young woman, and he cursed himself internally for thinking that. It was morally wrong for her to think that way, being 3 years older... but you were so attractive.