I see that questions about the usage of 'so' have been asked, but none of the answers I've seen get into why.
I first noticed this on the Shark Tank show. One of the sharks will ask a contestant something and almost always they begin the answer with 'so'. Since they come from all over America, I think they are being coached by the producers to do this. It bugs me, but I don't have a definitive reason why it is incorrect usage. In my dialect, 'so' at the beginning or within a sentence is equivalent to 'therefore'. (aside from it's meaning in sentences such as 'it's so much better that its in a different league')