Here's why:
* "Because" is a subordinating conjunction. This means it introduces a dependent clause, which can't stand alone as a sentence.
* Dependent clauses usually follow the independent clause. The independent clause is the main part of the sentence that can stand alone.
Example:
* Correct: I went to the store because I needed milk.
* Incorrect: I went to the store, because I needed milk.
Exceptions:
There are a few exceptions to this rule:
* If the dependent clause comes first. You'll need a comma to separate the clauses.
* If the sentence has multiple clauses and you want to add clarity.
Example:
* Correct: Because I needed milk, I went to the store.
* Correct: I went to the store, because I needed milk, and I wanted to pick up some snacks.
In general, though, it's safe to assume that you don't need a comma before "because."