Why was general lee asked to replace general johnston?

General lee was asked to replace general johnston, because during the beginning of the civil war General Lee worked as an advisor to the Confederate President, Jefferson Davis. In the spring of 1862 General Johnston was severely wounded and was forced to take a leave of absence from command in order to recuperate. In the meantime General Lee was asked by Davis to assume Johnston's command, which he did. In the time it took Johnston to heal, Lee had proven himself such an able andaggressive commander that Davis never restored Johnston to his former position.