"Why did G-d make our Patriarchs and Matriarchs childless? Because G-d desires the prayers of the righteous" (Yevamos 64a)
Now far be it from me to speak for the Mothers and the Fathers, but personally, I wouldn't want to worship an entity that is incomplete without my tefillos, and so covetous of having them that he goes and interfers with my wife's reproductive system just to get me moving. That kind of pique I can get from my children. It also seems rather blasphemous to ascribe such a devious motive to God, to suggest that He withheld a wish easily granted, and tormented His creations because their praise of Him was insufficient. Yet, according to many fundemnetalists, this is how God, Lord of the Universe, behaves.
But the more serious problem is this: God can't change. That's verbatam in the Torah, and ratified by the Rambam ( "...and He does not change, for there is nothing that can cause change in Him. There does not exist in Him... anger or laughter, happiness or sadness..." Yad, Laws Concerning the Fundamentals of our Faith, Ch.1 Law 11,) And if God can't change, why pray? Your prayers have no affect on his mood, or on his decisions. He can't be swayed, like a human can, through praises and supplications, so why bother?
One answer: "The answer is that the change that takes place through tefillah is not in G-d, but in ourselves. It is the same changeless G-d who treats the wicked one way and the righteous another way, the person who repents one way and the one who refuses to repent another way, the one who prays one way and the one who does not pray another way. Rambam gives an analogy. The same fire makes one thing black, another white, one thing hard, another soft...."[more]
Inspired by DBH