Shukr (acknowledgement) is a great virtue. If one lacks this virtue, he will be ungrateful towards man, and consequently, towards God Almighty. But Shukr is not a ready-made thing. We have to apply our intellect to discover it; otherwise, we will fail to be grateful towards both, man and God.
Here is an example to illustrate this point. When one enters a room and turns on a switch, suddenly, the room becomes full of light. Certainly, this calls for shukr; but people generally take it for granted, and therefore fail to acknowledge.