To say A is responsible for A is to say that A came before A, which is logically contradictory and therefore not a justifiable statement.
Agreed, however that's not how we would phrase the argument. It would make more sense to say that A has the "principle" of "A-ness" within itself. Therefore A explains A.
How this relates to God, creation, or anything of the sort, I'll give you a standard analogy.
Suppose we have a train car (Train car 1) in motion. And I ask you, why is train car 1 moving?
And you answer, because it's being pulled by train car 2.
I then ask you, why is train car two moving? And you answer because it's being pulled by train car 3.
This goes on for infinity.
If this were to go on for infinity, we never actually arrive at an explanation for why the train cars are moving. Because the "answer" just keeps getting pushed back one.
In order to explain the motion of the train cars, the buck has to stop at an entity that has the principle of motion within itself.
That's the engine car. The engine car has the principle of motion within itself, therefore, it is a sufficient explanation for why the train cars are moving.
In our train analogy, the cars are contingent entities. Their motion is explained by an entity outside themselves. The engine car is the necessary entity. It's motion is not explained by an entity outside itself, it's sufficient to move itself.
Now replace train cars and motion, for any given entity and existence.
Why does entity A exist? It exists because of entity B. Why does entity B exist? It exists because of entity C.
Again, if this goes on for infinity, we never arrive at an explanation.
We need to stop at an entity that has the principle of existence within itself. That, we would call God.
