Why is the following statment incorrect

[Raghuraman]

My teacher added a "because" after ,.

She was in ecstasy when she saw her math test, she received an A+.

[mykwyner]

You have two independent clauses (each can function as a sentence by itself).

You can join independent clauses with a semicolon: "Eat more vegetables; you'll live longer if you do."

You can use a subordinating conjunction: "Eat more vegetables because you'll live longer if you do.

You can use a coordinating conjunction and a comma: Eat more vegetables, and you'll live longer if you do.

If you use just a comma (as you did) it is a grammatical error called a comma splice.