[Yes, it should be a good memory and in its own way it is, but it's also tainted. The Doctor knows that the reasons for a lie are rarely ever so simple to explain. The same for one's own motives.]

But you lied to him anyway.

[He feels the scene begin to change, the smell of pizza fading; the Doctor closes his eyes again and instead sees the same man in Cassel's memory, the grin on his face, blackmail or a betrayal in the air, the impression of fear.]

He did find out, didn't he?

[Even as the Doctor wants to go deeper, to find out more because he very easily could, he knows he's already intruding. He mentally pulls back then removes his hands from Cassel.]

What made you more scared? Your brother or the consequences of your actions? [It's a question mildly asked. The Doctor isn't judging. He doesn't know all the details and it would be foolish to assume otherwise.]