Every one ought to apply himself to the duties suited to his character. Many, however, are hindered by following their fathers' pursuits. Clerics act in a different way.
224. Each one knows his own powers. Therefore let each one apply himself to that which he has chosen as suitable to himself. But he must first consider what will be the consequences. He may know his good points, but he must know his faults also. He must also be a fair judge of himself, so as to aim at what is good and avoid what is bad.
225. One is more fitted for the post of reader, another does better for the singing, a third is more solicitous for exorcising those possessed with an evil spirit, another, again, is held to be more suited to have the charge of the sacred things. All these things a priest should look at. He should give each one that particular duty for which he is best fitted. For whither each one's bent of mind leads him, or whatever duty befits him, that position or duty is filled with greater grace.
226. But as this is a difficult matter in every state of life, so in our case it is most difficult. For each one is wont to follow his parent's choice in life. Thus those whose fathers were in the army generally enter the army too. And others do the same with regard to the different professions.
227. In the clerical office, however, nothing is more rare than to find a man to follow his father's footsteps, either because the difficulties of the work hold him back, or continence in the uncertain days of youth is too difficult to hold to, or the life seems to be too quiet for the activity of youth. So they turn to those pursuits which are thought to be more showy. Most, indeed, prefer the present to the future. They are fighting for the present, we for the future. Wherefore it follows that the greater the cause in which we are engaged, the more must our attention be devoted to it.
Source: On the Duties of the Clergy (New Advent)