The Torah records that Hashem instructed Moshe to tell Aharon to initiate the first three plagues upon the egyptians; blood (Dam), Frogs (Tzifardeia), and Lice (Kinim). Why did Moshe rely on his brother to do these acts? Didn't Hashem appoint Moshe to go to Paroh?! Why didn't he initiate the plagues himself?
The answer is that Moshe wanted to express his appreciation for favors that were done for him in the past. All of the plagues that were just mentioned involved some damage to the forces that aided him in the past. The plague of Blood. caused the waters to become unusable. These same waters had once saved Moshe's life. Similarly, the plague of Lice was produced from the ground. That same dust protected Moshe when he buried the Egyptian that he killed. Therefore, because the waters and the ground had helped him during his lifetime, he gave Aharon the job of bringing about these plagues.
Gratitude is one of the pillars that sustains human society. Without it we would be very cold people. With it we would establish relationships with other people. By being grateful, one rewards those who had aided him. He shows that he recognizes the efforts rather than taking them for granted.