What do you think about this? I want to tell someone that I already know the reason why his girlfriend broke up with him because my boyfriend told me.
I don't see the point. Why do you feel the need to tell the guy what you know?
I think it's important for him to know.
You should wait for someone to tell you about their personal business instead of bringing it up. If you bring it up, it would make you appear to be intrusive and meddling into his private matters.
But why shouldn't he know that I know?
I just think it's better to stay out of other people's business when it comes to relationships. I don't think anything good could come from you mentioning it.