But you're trying not to be, right?

[And that, in his mind, makes all the difference.]

Even if you were like that in the past, and could be like that now...you don't want to be like that anymore, do you?

...You could have done way worse to me than a slap. But you didn't. You could be awful to everyone else here, but you aren't.

[He's not dumb - he sees things. He sees how Dazai interacts with other people, and even if their own relationship is rocky...]

...And I'd rather you be awful to me than to anyone else, because I can take it.