The "necessary being" argument states that as all things are caused then Nature itself must have been caused, and that it can only have been caused by something without causes - God.
Firstly, it may be true that all things have causes. But Nature Herself is not a "thing" and therefore cannot be said to have causes. "Things" can only exist for observers, and as we are manifestations of Nature, we cannot stand apart from Her to observe.
Secondly, no law says that all things must have causes. Show me a single cause! Show me where that cause begins and ends - it cannot be done, so why all this talk of "causes".
Thirdly, why create the notion of a "necessary being" at all? Such a being is actually an unnecessary being. Why must things have an ultimate cause? Why can't causes stem back endlessly?