The answer is simple; the explanation a little less so.
Basically, I fell into the habit of doing so in the early days of this blog (right at ten years ago now) and decided to retain it as distinctive, a part of my 'voice'. I'm also under the impression that I use it a fair amount in my daily speech.
The reason why I'm in the habit of using it is that I like the effect it gives of joining a conversation. Technically 'so' is a co-ordinating conjunction (like and, but, or, for, nor, and sometimes so and yet) yet I use it more like a conjunctive adverb (like however, nevertheless, on the other hand, &c). A sign of this is that when two sentences are joined with a co-ordinating conjunction, the conjunction is preceded with a comma.* But when two sentences are joined by a conjunctive adverb, the conjunctive adverb is preceded by a semicolon and followed by a comma.** Thus I'm using one part of speech but presenting it like another part of speech.
There's also the consideration that picky prescriptive grammarians disapprove of starting a sentence with a co-ordinating conjunction, despite the fact that the practice goes back a long way (it's endemic in the Anglo-Saxon Chronicle). So it's a small example of doing my part to reclaim a usage -- like ending a sentence with a preposition when that's the most natural word order.
Thanks to Clive S. for the query. And by the way, Clive, you're right in that well would be a very close parallel to my usage of so).
So, hope this helps.
*"He reads a lot of Tolkien, and also enjoys Dunsany."
**"He reads a lot of Tolkien; also, he enjoys Dunsany.