What makes a good and trustworthy listener?

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For some reason, plenty of friends (mostly girls, and I'm a guy) like to involve me with their problems, especially relationship ones. I never ask them to divulge their secrets to me, but for some reason I've got a few friends who just keep telling me their secrets. In the end, now I'm hiding their secrets from each other, even though all of us are mutual friends. It's kinda awkward at times, but other times when I think about it, it's kinda funny. :) So thing is, I'm wondering what kind of person would you willingly divulge your secrets to, even if they do nothing to ask about them?