The only suitable and proper reference for commanding or making a law (in religion) is Allah (SWT), for He is our creator and free of need from all others. He is therefore aware of what is good or bad for us. All commands issued by Allah (SWT) are based on wisdom. On the other hand, humans come to this understanding that many of their capabilities and affairs are out of their control and dependant on the grace and blessing of Allah (SWT). Without His grace and blessing, life will be impossible to continue. Therefore it is logical that man tries to obey the orders of Allah (SWT) submissively (even if he does not understand the underlying purpose of the orders.)
As we explained earlier, there are definitely deep underlying purposes for every command Allah (SWT) has ordained upon us; many of which we may not be able to understand. So the answer to the question of why Muslims pray five times a day lies in obeying Allah's (SWT) command. It should be noted that according to wisdom, submissive obedience is only towards Allah (SWT), and such obedience has shown to have many useful effects on man.
Stating life examples may further explain this issue:
1- We have all one time or another seen a doctor to cure an illness. To cure us, the doctor prescribes certain medicines. However, have we usually taken the time to ask the doctor the theories and exact reasons behind each single medicine? Certainly not! This is simply because we trust the doctor; we know that understanding the reason(s) behind a medication requires medical knowledge, which we may not have. Thus we accept the doctor's prescription without hesitation. (Of course it may happen that the doctor simply explains the reasons for prescribing a certain medication for us.)
2- Assume that we are given a treasure map. The map says that in order to find the treasure, you must start from point X, take five steps to the North, and then 6 steps toward the East. If we do not follow the path exactly, we will not be able to find the treasure. In the same manner, if we do not follow what Allah (SWT) has ordered us to do, then we are definitely not going to reach the sacred destination that can make us successful in both worlds.