Here's why:
* "But" is a coordinating conjunction. It connects two independent clauses (clauses that could stand alone as complete sentences).
* Coordinating conjunctions like "but" are typically followed by a comma only if the conjunction is not at the beginning of the sentence.
Example:
* Correct: The weather was awful, but we had a great time.
* Incorrect: The weather was awful, but, we had a great time.
However, there are some exceptions:
* When "but" is used to introduce a contrast between two phrases that are not independent clauses. For example: "I wanted to go to the beach, but, knowing it would be crowded, I decided to stay home."
* For stylistic reasons, sometimes a comma is used before "but" to emphasize a contrast or create a particular rhythm. However, this is more common in creative writing than in formal writing.
In most cases, it's best to stick with the general rule and avoid using a comma before "but."