I don’t know what this means?

my male friend just pointed out that I relate to him so well that when he says something in slang I am able to relate to it and communicate it in regular English. He was surprised to the extent that he told his sister that we were similar. What does this mean?

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The thing is that we are friends and it’s a given that for us to be friends we need to have something in common. Why is he making so much emphasis on us being similar?
I don’t know what this means?
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