Here's why:
* "Because" is a subordinating conjunction. It introduces a dependent clause that can't stand alone as a complete sentence.
* Dependent clauses need to be connected to an independent clause (a complete sentence) with a comma.
Example:
* Correct: I went to the store, because I needed milk.
* Incorrect: I went to the store because, I needed milk.
Let me know if you have any other grammar questions!