Charlie isn't going red, or looking like he's about to be ill; he's not yelling or threatening. All the same, Ned feels he ought to preface his next question with, "If you don't want to answer me that's fine, just tell me to shut up and I will. I guess I'm just curious... how it happened. And I thought, I mean, since it's a secret, you might not have ever talked about it before, and you might want to."

It might not be exactly the same as the secret that he'd been carrying around with him those twenty years, but Ned knows that, when the first person (River, he thinks) found out, it had been surprisingly nice to just.. talk about his powers, on a very basic level.