Dean Thomas is confused, and that scares him. He usually always knows what to do, he is normally calm and collected. He is everybody's anchor. This time, though, it's different and he doesn't know what to do about it. He doesn't know how to feel.
What he does know, though, is that everytime he wakes up from a nightamre, he is grateful to have her beside him in the dark, offering essence of gurdyroot for his dreams. He knows that when Seamus and Lavender come over for supper, he is proud to have her by his side, commenting dreamily on the conversation. Sometimes he gets wrapped up in the feel of her soft hand in his, the look her white and his black, contrasting beautifully. He thinks she is sweet and different, the complete opposite of him, who throws the scattered artist stereotype to the wind. Dean is happy, happier than he ever was with Ginny or Padma or Hannah, and he is frightened.
He is afraid because what if he loses her, or worse, loses himself? He is not as strong and brave as Seamus, or as centered as Hermione. He is afraid of losing his sanity, and Luna can't even help him. What if one day, he wakes up and his love is gone, having drifted off to her next adventure, the next chapter of her life? What if the pages will never turn again for Dean, and she will be on to the the next paragraph while he is still desperately rereading the first sentence?
He's never felt like this before, like he could lose control at any moment. He didn't feel that way when he ran away from school, he didn't feel that way when his little sister was killed by a Death Eater, and he didn't feel that way when his mother, his rock of a mother, was diagnosed with cancer. That's why he wants to make sure she loves him. He's had an idea, and it is resting in his pocket right now, safe in its little velvet box.
Maybe he is more sure of everything than he thinks. Maybe he shouldn't be afraid. Because he loves her, and hopefully she loves him back. That, he thinks, is enough for now.