What's it mean? What's "you" doing?

Native speaker’s answer
Rebecca
Here, the line "You doing that thing you do" means that this person is acting or doing the thing they usually do. It could imply any action, really, but it's one they do often. Here, it links to the next line, "breaking my heart into a million pieces." Implying that what this person usually does is break her heart. In other contexts, "You doing that thing you do" could imply that whatever the person is doing or however the person is acting, they like it and find it attractive. Ex: You're doing that thing you do when you're nervous. Ex: He keeps doing that thing he does. And now I can't get him off my mind.