Imam expains the matter in his famous book "an exposition on Forty Hadith" as following:
You know that thankfulness is natural and man’s nature commands man to be grateful to his benefactor.
If one tries to read the book of his own heart, he will see that this law is written there. The sense of veneration and gratitude for the benefactors increases with the amount of benefaction, particularly if the benefactor’s generosity has no selfish motive.
Greater the selfless generosity, greater is sense of gratitude. For example, compare the extent of veneration for one who presents you a horse with a selfish motive, with the respect you have for a person, who grants you a village of several hectares, without any inkling of selfish motives.
If a doctor rescues you from the darkness of blindness, you naturally owe to him a lot of respect, and if someone saves you from the clutches of death, you owe him a lot more.
You yourselves reflect and estimate the seen and unseen favors bestowed on us by the Almighty, even a small fraction of which all men and the jinn cannot even provide us with.
Take for instance the air we breathe day and night, upon which our existence as well as that of other living beings is dependent, nothing can remain alive even if it is taken off merely for a quarter of an hour.
What a marvelous gift it is! If all men and the jinn of the world labor hard to contrive such a thing, they cannot do that. In the same way, try to recall other gifts of God also, like external bodily senses viz. sight, hearing, taste, smell and touch, etc., and inner faculties like thinking, imagination and reason, etc., each one of which carries unlimited benefits and which are granted to us by Our Lord. Besides these valuable gifts, there are several other gifts also.
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