This is really not true. Watching my niece grow up from birth has proven to me that some people are simply outgoing. She didn't learn it, she just was it from the get go. maybe others learn to be shy and then have to spend a lot of effort unlearning it. and of course there are many people are are naturally shy. But not by any means all of us.
(I think the general term for this is the typical mind fallacy: we all model others by generalizing from ourselves. It's usually a pretty reasonable way for our minds to work; after all, as humans we are much more alike than we are different.)
