Belief in God is not a proposition to which the word "why" can legitimately be applied.

By definition, God is limitless. The very act of attempting to explain why one should believe in God treats God as something which can be defined in human terms. But anything that can be defined in human terms is not limitless and, therefore, not God.

So, the best someone who attempts to answer that question can do is tell you why you should believe in something that, while it may have some God-like attributes, isn't really God at all.