By Rabbi David Cohen:
When Hashem first appeared to Moshe and assigned him the task of leading Bnei Yisrael out of Egypt, Moshe initially refused. One of the reasons he gave for his refusal is that he suffered from a speech impediment, which compromised his ability to effectively confront Pharaoh. Hashem replied that he should nevertheless approach Pharaoh, and his brother, Aharon will serve as his spokesman.
Ramban raises the question: why did Hashem not simply cure Moses’ impediment, rather than have Aharon speak in Moshe’s place? Ramban answers that Moshe did not ask to be cured.Hashem was prepared to provide the cure, but withheld it because the request was not made. We learn a tremendous lesson. Asking is necessary and required to bring down the bracha from Shamayim.