Musar su Salmi 91:15

When he responded, Isaiah only volunteered for the שליחות, G–d's mission, not for the הליכה, which could have passed for his own initiative. G–d had to tell him then that at that point in time He needed a messenger only for the unpleasant part of the mission. This is why He said to him לך ואמרת, "go and say." Similarly, in our case, G–d wanted to provoke Pharaoh so that out of the additional "darkness" the contrast with the "light" of the redemption would be so much greater. In other words, יהי שם השם מברך, first afflictions, then peace, serenity. G–d Himself is with us even while we suffer, i.e. His own name will become greater only in the future. This is the meaning of עמו אנכי בצרה, "I am with Israel when it is in trouble" (Psalms 91,15). There is no greater trouble than that G–d's Presence must exile itself on account of our sins. In Exodus 1,14, וימררו את חייהם בעבודה קשה, "They made their lives miserable with hard labour," the word חייהם refers to that which gives us our strength, i.e. G–d. In other words, the Egyptians made our lives in the Celestial Regions bitter.

G–d's question in 14,15: מה תצעק אלי, which Rashi had understood as a criticism of Moses who engaged in lengthy prayer when the need of the hour was action, becomes clearer now. G–d hinted to Moses that he was mistaken when he thought that he fulfilled G–d's desire, i.e. that his prayer was something which G–d "needed." As long as Israel was in distress, i.e. in exile, not on the soil of the land of Israel, there is no such thing as G–d "desiring" or "needing" such prayer. As long as the שר של מצרים is still in pursuit of the people of Israel, "I may be with My people, as I am always when they are in difficulties, but I do not 'desire' their prayer." At a time such as this the adversary has to be humbled; this can only be done by action. Hence: "tell the Israelites to get moving!"