Why do Muslims have to pray Zuhur 4 Rakaah, Asr 4 Rakaah, Maghrib 3 Rakaah, Isha' 4 Rakaah and Fajr 2 Rakaah?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

To answer this question one needs to know the meaning of the word "Islam" itself from which the word Muslimoun was derived. In fact, Islam means the submission and the complete obedience to Allah, The Almighty. So, Muslims are those who abide by Allah's Orders and comply with them without any reservations, even if they do not know the wisdom behind them, as this is what Islam actually is. Know, however, that behind each Order of Allah there is a great wisdom that might be known to some people but unknown to others. Our ignorance of the wisdom behind a given Order does not mean that there is no wisdom behind it; it rather means that we are short of knowing the Wisdom of Allah. I come back to the question and say: "The prayer brings happiness to the hearts. It strengthens them and makes them feel delighted while talking directly to Almighty Allah and standing in front of Him, using the whole body in worshipping Him and showing submission to Him with each and every part of one's body.

Allah ordered us to pray five times during the daytime and the night. He prescribed appropriate times for these prayers in order to establish a permanent relation with His slaves, who ask for His Help, Sustenance, and Guidance to the Right Path. There is much other wisdom.

Allah knows best.