All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
Any thoughtful and reasonable person who knows Allaah's Greatness and Might would not ask such a question. Indeed such a person does not value Allaah as highly as He should be valued. This led him to oppose Allaah's Law and preordainment regarding His Creatures as well as Allaah's Will in managing His Kingdom.
When talking to such a person, you should first discuss belief matters through Allaah's great Signs in the universe, emphasizing the extraordinary nature of the creation of this universe. If this person becomes Muslim and the right belief enters his heart, this will bear the fruit of obedience and submission to Allaah in his heart. He will understand subsequently that when Allaah decrees something, no one has the right to object to it and if Allaah decrees, no one can turn away His decree. That person will realize undoubtedly that he is weaker than to judge Allaah's Actions whether or not they are merciful.
If this man believes in Allaah, he will know that this life is but a transit to the eternal life and that this life is a life of trials and temptations and that the extent of the reward is relative to the extent of the hardship. Now if Allaah has tried women by making them menstruate, get pregnant, deliver and breastfeed, He has also greatly rewarded them for the hardships faced during these matters.
In Islam, a child is asked to allot three-quarters of his dutifulness and good company to his mother and only one-quarter to his father.
Al-Bukhari and Muslim reported that a man came to the Prophet and said: “O Prophet of Allaah who amongst all the people is more deserving of my good companionship?” The Prophet said: “Your mother?” The man said: “Then who?” He replied: “Your mother” He again asked: “Then who?” The Prophet again replied: “Your mother” The man asked again: “Then who?” Thereupon, the Prophet said: “Your father.”
This is an aspect of the Mercy of Allaah towards mothers. On the one hand, if the mother remains patient while undergoing such difficult moments in menstruation, pregnancy and delivery, she gets great rewards from Allaah, The Greatest.
Al-Bukhari and Muslim reported that the Prophet said to ‘Aa’ishah during her ‘Umrah: “But it (your reward) is proportional to the hardship you encounter.”
On the other hand, why didn’t this man reflect on the fact that Allaah did not oblige women to pray while menstruating and did not ask them to make these prayers up. Why didn't he think about the fact that Islam does not make it compulsory on women to perform many other hard tasks, like working as hard as possible to attain their expenditures.
Anyway, it is impossible for any person to realize the wisdoms that Allaah has in His Creation and His Laws. It is enough for the person to believe in Allaah's Wisdom and Knowledge and in His Kindness and His Mercy to His Creatures. Also, the person should realize his incapability to realize the secrets of Allaah and submit himself as a result.
Allaah Knows best.