It’s quite well known that the first three plagues were actually performed via Aharon, and not Mohse. This is because, as Rashi points out (7;19 & 8;12)the plagues of blood and frogs required the hitting of the nile river and the plague of lice necessitated hitting the dust of the earth.
Now since Moshe was saved by the Nile and the earth dust was used by him in burrying the Egyptain officer he killed, he was to have hakaras hatov (grattitude) to the Nile and dust, and so it was to be Aharon who hit them, not Moshe. But there’s a central question here:
Why does Moshe need to have hakaras hatov to inanimate objects like a river or dust; they have no feelings nor appreciation of kindliness?
The answer is that hakaras hatov is not centred around the receiver of the hakaras hatov, but is rather a character trait that should be built up and living inside of us regardless of who the receiver is.
Thus, Moshe was to have the middah of hakaras hatov so ingrained into his personality that it even expressed itself towards inanimate objects.
