Gemara:


Ein Ayah: Although prayer is based on emotion, it also has to be influenced by logic, to the degree that the emotion needs to be attached to reality. It is true that the goal and the workings of prayer are to improve a person spiritually. [This is a thesis that Rav Kook discusses in many places – sometimes Hashem allows a person to receive that which He has destined for him only by means of his praying for it, in order for this experience to attach the person to Him]. This is why the ultimate wisdom arranged in advance that as everything has a correct time, so too will matters arise that will motivate a person to pray for certain needs. The moral lift for a person’s personality that he will receive through prayer will thereby fit the general reason that Hashem created the phenomena related to prayer and will justify the person’s specific request being accepted in the desired manner.
For this process of prayer to be of proper value, two conditions must be fulfilled in regard to the nature of the request. One is that the request must be accomplishable by natural means with which the world functions. If this is fulfilled, then prayer becomes a link in the chain that joins the existence of the world along the lines of wonderful order and divine wisdom. The other condition is that the prayer has to follow lines of healthy logic. In such a way the prayer itself improves the internal spirit and at times creates a situation where it is appropriate for his specific request to be accepted by Hashem. When a prayer is either missing conforming to the nature of the world or is an illogical request, it will not bring the desired closeness to Hashem.
The two worthless prayers spelled out by the mishna are lacking in the two aspects that we have described, respectively. The prayer that the pregnant woman will give birth to a boy is against nature. [This is because it would be of value only if the fetus was a girl and was changed due to the prayer into a boy, which would of course be against the rules of nature]. The prayer about the source of the cries of anguish is not even logical, as it about something in the past. The fact is that the cries emanated from one place or another, and the prayer cannot change that which already happened. Such prayers that do not follow the mode of proper requests not only will not be answered, but do not even provide a person with the experience of prayer in which his spiritual being can be improved. That is why they are called worthless prayers.