Guys, Why doesn't he tell me himself?

I'm talking to this guy whose in a relationship. We have developed a really strong bond with one another, so we talk about everything. However, I became very cool with his bestfriend to where now he calls me "sis". One night his friend decides to tell me how the guy really feels about me, which confuses me because he never tell me these things. Why is he telling his friend but not me?

Guys, Why doesn't he tell me himself?
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