The catalyst for me was realising how good a writer I truly was. I was on the panel that read the first submissions for an Anthology of short stories a group I am involved in decided to do. When I read those submissions, and some were from self-published writers, I really had to evaluate my own writing and discovered I was way up the top of the list.
Since then I have taken some giant leaps. It is a real boost (to the ego, morale, and self confidence,) when three of your bosses buy a book each.