Here's why:
* Clarity: Starting with a conjunction can sometimes make the sentence feel choppy or unclear.
* Emphasis: Using conjunctions at the beginning of sentences can sometimes shift the emphasis away from the main point of the sentence.
* Formal tone: In formal writing, starting sentences with conjunctions is often seen as less sophisticated.
That being said, there are exceptions to this rule:
* Emphasis: You can use "but" at the beginning of a sentence to emphasize a contrast or to create a dramatic effect.
* Dialogue: In dialogue, it's perfectly natural to start sentences with conjunctions.
* Informal writing: In informal writing, you have more freedom to use conjunctions at the beginning of sentences.
Ultimately, whether or not to start a sentence with "but" is a matter of style and context. If you're unsure, it's always best to err on the side of caution and avoid it in formal writing.