Sam winced slightly, part of him wished she hadn't asked because then he wouldn't have to explain. He clasped his hands in front of him and gave some consideration to how to word his reply, but he struggled to come up with a response that wouldn't sound crazy.
"Something happened. Something that put both you and Ben in danger, because of the work that Dean and I do. Dean couldn't live with it, so he thought it was better and safer for you both if you had no memory of him, or me or anything about us." Sam sighed, unable to find a way to explain the next part without it sounding crazy. "Ready to feel like the sane one again? He asked an angel to alter your memories."