Why ONE God only ?

Can any of you give a satisfying and convincing answer as to why there can only be one God, not two or several ? To say that if there will be more than one God, they will fight, is a useless argument because Gods are not like human that they'll fight, if they can do 'everything' then they can also be nice to each other.

Islam says there is one God, but still, some people believe in monotheism and some don't. If there would have been several gods, still, some would believe that there is only one and some would believe there are more, because people may never know the reality that how many gods exist. It is only the definition of a God which either decides or rejects the concept of monotheism. The definition of God in Islam, or as given by Hazrat Ali, clearly highlights a logical reason as to why there cannot be several gods.

"To believe in His oneness is to realize that He is so absolutely pure and above nature, that nothing can be added to or substracted from Him; because one should realize that there is no difference between His Person and His attributes. Whoever accepts His attributes to be an addition to His Being abandons the concept of monotheism and beleives in duality (He and His attributes). Such a person, in fact, believes Him to exist in parts. 

One who holds such a belief, cannot form a true concept of Allah. He is ignorant and will always try to believe in some creation of his imagination as his god. And whoever holds such a belief accepts limitations in His Being and confines Him to a place or to particular attributes and brings Him on a level with His creatures. 

 To believe that He is invested with the characterstic of residing in a particular place(Heaven or earth) and to point towards Him accordingly, or to believe that He is restricted to a special position or occasion, or to consider that certain place or time can exist without Him, and can be void of His Omniscience and omnipresence is a conception which results in assigning Godhead to a numerical unity. 

 To assign a place to Him by considering Him within or over a place means to subject Him to the limitations of space and allot Him an importance secondary to space; it also means to believe that some place can exist outside the sphere of His Omnipresence. His existence is eternal; He neither came into being at some period of time, nor was He created. His being did not come into existence from nonentity. He is with everything but not physically and bodily; He is away from everything but not by way of bodily distance or of being unconcerned and indifferent to it".

In short, Allah is everywhere, He is with everything, He exercises power over everything, He controls everything. Let us suppose there are two Gods(A and B), then both of them cannot reside 'everywhere' as they are two separate existences since A cannot be within B, and B is not in A so these Gods are not everywhere. 

Similarly, God A exercises absolute power over the universe, but if it doesn't exercises power over God B then A is not a perfect God as it does not have power over EVERYTHING. (And in case A can be within B and they both exercise equal power over each other and have control on each other then, first of all its not possible, and secondly, it would mean that A=B which would mean that God is in fact, only One). 

 So, it is not said that Allah is 'everywhere in the universe only' or that he controls everything 'just within this universe'. Everything MEANS everything and Every where means EVERY WHERE (outside universe as well). While this universe is limited and Allah has confined its limits and He Alone is the God of each and everything.