I think it's very possible that he just wanted to share a happy memory with you.
That being said, I understand the feeling that you let him down because you couldn't pick up on his strain and secrecy... that you were so close to him, you should have known, etc.
Don't underestimate how good survivors get at keeping those secrets. Honestly, if he does feel that you let him down by being unable to read his mind... that's a perfect example of the "not taking responsibility" that we were discussing in the other thread.
Why didn't I know for so long? Because he made a decision long before I came around, that his survival depended on not telling anyone.
Take care of yourself,