I don't think people are introverted because of rejection. It's just the way Allah Ta'aala has made them. Allah has made some people more recluse than others, and some Allah Ta'aala has made very sociable. Both are commendable and do not have to change, as long as it does not hinder their Shar'ee duties. What sister Acacia mentioned was true that you cannot figure out who are introverts or extroverts through forums. I personally know some on the forum who may seem like extroverts from their posts, but if those articles I posted are anything to go by, then those people would be considered ideal introverts.