So apparently, our use of the world "so" is changing. It's sneaking into the beginning of more and more of our sentences, where it can denote many things: a conclusion, a transition, a flash of insight. Anand Giridharadas speculates on the reasons for its rise:
To answer a question starting with “well” suggests you are still considering it, don’t know fully but are getting there. To answer with “so” better suits the age, perhaps: A Google-glued generation can look it up where their parents would have said “I don’t know,” Facebook and Twitter encourage ordinary people and not just politicians to stay on message, and we gravitate toward declamatory blogs and away from down-the-middle reporting. “So” also echoes the influence of a science– and data-driven culture.
But that's just one explanation; there are others.