Allah tells us that He chooses Messengers from His angels as He wills by His law and decree, and He chooses Messengers from mankind to convey His Message.
(Verily, Allah is All-Hearer, All-Seer.) means, He hears all that His servants say, and He sees them and knows who among them is deserving of that, as He says:
(Allah knows best with whom to place His Message) 6:124
(He knows what is before them, and what is behind them. And to Allah return all matters.) He knows what will happen to His Messengers and the Message He sent them with, for none of their affairs are hidden from Him. He says:
("The All-Knower of the Ghayb (Unseen), and He reveals to none His Ghayb.")72:26 until His saying;
(and He keeps count of all things.) 72:28 So He, may He be glorified, is guarding them, and is Witness to what is said about them. He is protecting them and supporting them.
(O Messenger! Proclaim which has been sent down to you from your Lord. And if you do not, then you have not conveyed His Message. Allah will protect you from mankind) 5:67.