I ask myself that same question and thought of making a thread asking how to approach girls I don't know. I'm in a situation right now where I want to talk and approach a girl I like that I don't know but I have no idea what to say to her. It just baffles me how people can do this with ease and can think of what to say on the fly. I guess the big secret it to just not care anymore. If she rejects you, then it's over. You won't have to keep wondering anymore. I will have to apply that myself. But for me it's not the asking out part that's the hardest, it's that opening line and what to say to start it.
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"The less I know about other people's affairs, the happier I am. I'm not interested in caring about people. I once worked with a guy for three years and never learned his name. The best friend I ever had. We still never talk sometimes."