Here's why:
* "But" is a coordinating conjunction. It joins two independent clauses (sentences that can stand alone).
* Coordinating conjunctions generally require a comma before them. This helps to separate the two clauses and improve readability.
Example:
* I love to eat ice cream, but I know it's not good for me.
Exceptions:
There are a few exceptions where a comma isn't necessary before "but":
* Short, simple clauses: If the clauses are very short and simple, a comma might be optional. For example: "I went to the store but forgot my wallet."
* Emphasis: Sometimes, omitting the comma before "but" can be used for emphasis. For example: "I tried to warn him, but he wouldn't listen."
However, in most cases, it's safer to err on the side of using a comma before "but" to ensure clarity and proper grammar.