Why did my guy friend say I know him well, maybe even too well?

Anonymous
We know each other for 8 years, and we are close friends for over 4 years. We are both married and nothing ever happened between us. We both know it wouldn't be so if we were single.
He has a lot of people around him, but not any friends. He always brags about how no one knows him more than he let them know.
I started telling him things about himself that I noticed and he began to trust me and confide in me.
When we joke, he still says that I know only things he let me know about him, but when we’re serious he confesses that I was right about everything I said about him.

Why does he think it is a bad thing to know him so well? I proved to be a confident person who can be trusted.
Why did my guy friend say I know him well, maybe even too well?
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