One of the main reasons people become scientists, which they're not supposed to talk about, is to satisfy their ego. I have become a scientist, and I can attest that the ego is indeed fairly well satisfied. But, it seems, not quite enough. Although I have convinced myself that I am extremely clever, and that the work I do is so important that I don't mind being paid relatively little to do it (certainly much less than a brilliant mind like mine could presumably earn if turned to a lesser vocation), my ego still wants more. Fame! Glory! The adoration of millions! Unfortunately, having spent all my adult life training to be a scientist, it's a little late to become a rock star or a Hollywood celebrity. Plus, see lesser vocations, above.
A blog is the perfect option. It is a natural extension of the scientist's urge to continuously pontificate, and my observations of the internet suggest that notoriety can be achieved even if you have nothing original or useful to say. The internet has done a sterling job of uncovering a great democratic urge to communicate the fathomless depths of inanity within us all --- so I'm sure I have plenty to offer.
In most hit blogs the author is either an entertaining screw-up, or screws entertainingly. Scientists are invariably entertaining screw-ups, so I have that one covered. I'll have to remember to change the names and shuffle around the facts, but as a practiced scientific researcher, that will come naturally. As for the second point: forget it. If the world's labs, libraries and lecture theatres constitute a modern-day re-enactment of the sauciest passages of the Decameron, then they've kept it all well hidden from me. I could make something up, but this isn't that kind of blog.
The ideal blog is written by an expert we're thankful to hear from. That might work. I can occasionally feign being an expert. After all, I do know a little bit about physics. I can say nice things about black holes and rude things about string theorists as well as the next guy.
Besides self aggrandisement, there are slightly more serious reasons (i.e., not made up as an excuse for flippant remarks) why I have started a blog. But since it is the nature of blogs to be brief and dashed-off, I will leave those for next time.