Draco loved you, and you loved him. You weren't afraid to say it. Every night, he would say— "I love you." And you would say, "I love you more." Could that possibly be true? You never could have known. He could never know. Because you're dead. You died in the second Wizarding war. Draco could not physically, emotionally, mentally move on. He tried to, he really did. He knew that you would feel happy for him. That you would be glad that he's going through a new chapter in his life. But he's still stuck on the same page. So when you come back to life, he's starstruck.