In short: Given the seriousness of the pastoral office, those who are unqualified or lusting for glory should not enter it. To profane it leads to perdition. Jesus “fled from the offered glory of pre-eminence, but desired the pain of an ignominious death; that so His members might learn to fly from the favours of the world, to be afraid of no terrors, to love adversity for the truth’s sake, and to shrink in fear from prosperity.” The school of adversity usually benefits the heart and mind far more than elevation to high office, which often brings trouble. Both Saul and David suffered severely. – CSB
Of the weight of government; and that all manner of adversity is to be despised, and prosperity feared.
So much, then, have we briefly said, to shew how great is the weight of government, lest whosoever is unequal to sacred offices of government should dare to profane them, and through lust of pre-eminence undertake a leadership of perdition. For hence it is that James affectionately deters us, saying, Be not made many masters, my brethren (James iii. 1). Hence the Mediator between God and man Himself—He who, transcending the knowledge and understanding even of supernal spirits, reigns in heaven from eternity—on earth fled from receiving a kingdom. For it is written, When Jesus therefore perceived that they would come and take Him by force, to make Him a king, He departed again into the mountain Himself alone (Joh. vi. 15). For who could so blamelessly have had principality over men as He who would in fact have reigned over those whom He had Himself created? But, because He had come in the flesh to this end, that He might not only redeem us by His passion but also teach us by His conversation, offering Himself as an example to His followers, He would not be made a king; but He went of His own accord to the gibbet of the cross. He fled from the offered glory of pre-eminence, but desired the pain of an ignominious death; that so His members might learn to fly from the favours of the world, to be afraid of no terrors, to love adversity for the truth’s sake, and to shrink in fear from prosperity; because this often defiles the heart through vain glory, while that purges it through sorrow; in this the mind exalts itself, but in that, even though it had once exalted itself, it brings itself low; in this man forgets himself, but in that, even perforce and against his will, he is recalled to memory of what he is; in this even good things done aforetime often come to nothing, but in that faults even of long standing are wiped away. For commonly in the school of adversity the heart is subdued under discipline, while, on sudden attainment of supreme rule, it is forthwith changed and becomes elated through familiarity with glory. Thus Saul, who had before fled in consideration of his unworthiness, no sooner had assumed the government of the kingdom than he was puffed up (1 Kings x. 22; xv. 17, 30); for, desirous of being honoured before the people while unwilling to be publicly blamed, he cut off from himself even him who had anointed him to the kingdom. Thus David, who in the judgment of Him who chose him was well pleasing to Him in almost all his deeds, as soon as the weight of pressure was removed, broke out into a swelling sore (2 Kings xi. 3, seq.), and, having been as a laxly running one in his appetite for the woman, became as a cruelly hard one in the slaughter of the man; and he who had before known pitifully how to spare the bad learnt afterwards, without impediment of hesitation, to pant even for the death of the good (Ibid. 15). For, indeed, previously he had been unwilling to smite his captured persecutor; and afterwards, with loss to his wearied army, he destroyed even his devoted soldier. And in truth his crime would have snatched him farther away from the number of the elect, had not scourges called him back to pardon.