Salaam
Consider why a person may love another person... Lets look at the example of the love between mother and child. The reason why the child loves his/her mother is due to the care, mercy, affection, nurturing [etc] of the mother. If these things were not done to the child, he probably would not love his own mother...
So from this simple example, we can say that the reason we love God is because He created us and gave us countless amount of blessings - including the mother. He gave us eyes, ears, mind, health, wealth, friends...every person has some sort of blessing. So Allaah should be loved, and is ever deserving of it, for His favours upon mankind; for His Merciful nature and also for the Mercy He has shown to us as an individual, specifically.
And also, no matter how merciful and caring the mother is, she can never be more caring and merciful than Allaah, thus not only should we love God, He deserves a higher level of love than anyone else...
Hope I was able to give a presentable answer... post back if you got any more questions