'Only' word is important from another angle.
God is divine as opposed to to our mind which is material.
The material mind is incapable of seeking divine happiness.
Materiality of material mind can be washed off only by divine material. As divine personality enters mind, material mind begins to shed its material character. The more the divine intake, the purity of mind is on increase while material character of mind is on decrease in the same proportion.
Which means the material intake increases material character while divine intake makes mind empty of material character.
Given that mind must be 100% pure or 100% empty for its transformation into divine, there has to be continuous divine intake because any material intake will make mind more impure.
Hence only divine thoughts of God should enter our mind. It is possible only if 'ONLY' word is used. Because moment we use 'too' word, material intake is on which takes us away from purified state of mind.
Thus shed 'too', accept 'only' and purify the heart as fast as possible to get entry into God's divine world as early as possible.
Because mind has habit of having all relations since time infinite, God says that all relations should be established with him. By doing so mind does not find any need to fall back on material persons for fulfillment of any relations. In addition, God being all mighty he can fulfill all relations in true manner. Hence God is souls only father, only mother, only brother, only sister, only friend, only husband, only well wisher, only teacher and so on.