So....

[Ridgley]

Really Lisa :)

[Nigello]

The "so" at the start of a sentence allows the speaker to be dominant because its use suggests that s/he has knowledge of what is about to come whilst the interlocutor does not. However, I don't think for a moment that most of the current "so" users do it for that purpose: they're just following fashion in the same way as those who say "gotten" or "here's the thing" are.

[Grok]

The So at the start of every sentence, mainly used by youth and oldies trying to act youthful is to make up for lack of basic education, to give their slow brain fuction time to catch up.

[FJDGoose]

Beginning a sentence with "because" is irritating, there's a ad on the radio at the moment that does it

[I hammer]

Basically.

So lah tee doh.