That depends on the person. If they aren't inclined to be social there's not much you can do about it, really; but if they're merely shy, try showing them people aren't as scary as they make them out to be. Also works if you show them that some people are just as scared of them as they are of other people. Find someone who shares their interests, someone whose personality you think would mesh well with theirs. Someone they can meet on an intellectual level.
As for hobbies--well, that's a bit more unpredictable. Sometimes all you have to do is shove one in their hands and they'll pick it up like they were born to do it.
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As for hobbies--well, that's a bit more unpredictable. Sometimes all you have to do is shove one in their hands and they'll pick it up like they were born to do it.