"You must not start a sentence with a conjunction" has been the mantra of many an exasperated English teacher, especially since the oiks started getting state education. Any pupil brave, daft or naive enough to raise a hand and ask "Why?" would probably have received some Kafkaesque explanation along the lines of "Because it is said."
So now we're all grown up, we'll ask again - why? The honest answer is that there is no reason. It's just an arbitrary rule that's been passed through the ages. But all language is arbitrary, isn't it? Words and letters are but abstract concepts, the understanding of which can only come through the education of rules.
As usual with these issues, the argument is between the way English is used and the way it is "supposed to be". Sometimes a full stop is required to drive home a sentence or give a meaningful pause. And an afterthought like this one gets more impact when it's a sentence rather than a conjoined clause. So when someone tells you not to start a sentence with a conjunction, say: "But I've already done it."
http://www.gpuss.co.uk/english_usage/start_sentence_conjunction.htm Oiks?