She doesn't stop him from wrapping his arm around her waist. She wants to, though. Tris had stepped away from him for a reason. It's because she had come here initially to be alone and the direction her thoughts had just taken reminded her of that. His arm around her waist reminds her that she isn't alone and of all the secrets she's still keeping from him. He had come clean about her parents' house being here. Why can't she do the same?

She really isn't as brave as she wants to be.

His question stings about as much as the nick of a knife would and it's enough to make her finally pull away. She shakes her head in response as she folds her arms up against her chest. "I just want to be alone."