While some grammar guides from decades ago discouraged this, it's now considered perfectly standard and natural.
Here's why:
* Emphasis: Starting with "but" often creates emphasis and can add a sense of contrast or surprise.
* Flow: It can create a smooth flow between ideas, especially in transitions between paragraphs or sections.
* Natural language: In everyday speech, starting sentences with "but" is common and often sounds more natural than avoiding it.
However, it's still wise to use "but" strategically. Too many "but" sentences in a row can make your writing sound choppy.