I wouldn't even say it's a challenge to talk to strangers. I can do it fine, and do it all day at my job. It's just, I don't feel the desire to do so simply for the sake of doing it. If I'm sat in a park reading a book on a Saturday morning, I can pretty much guarantee you I do not want to talk to people, yet you'll get people who just want to plop down next to you on the bench, let out a sigh and say "Hoo, boy what a week, eh?," and expect you to put down what you were doing to keep the conversation going.