Objection 1. It seems that ambition is not a sin. For ambition denotes the desire of honor. Now honor is in itself a good thing, and the greatest of external goods: wherefore those who care not for honor are reproved. Therefore ambition is not a sin; rather is it something deserving of praise, in so far as a good is laudably desired.
Objection 2. Further, anyone may, without sin, desire what is due to him as a reward. Now honor is the reward of virtue, as the Philosopher states (Ethic. i, 12; iv, 3; viii, 14). Therefore ambition of honor is not a sin.
Objection 3. Further, that which heartens a man to do good and disheartens him from doing evil, is not a sin. Now honor heartens men to do good and to avoid evil; thus the Philosopher says (Ethic. iii, 8) that "with the bravest men, cowards are held in dishonor, and the brave in honor": and Tully says (De Tusc. Quaest. i) that "honor fosters the arts." Therefore ambition is not a sin.
On the contrary, It is written (1 Corinthians 13:5) that "charity is not ambitious, seeketh not her own." Now nothing is contrary to charity, except sin. Therefore ambition is a sin.
I answer that, As stated above (103, 1 and 2), honor denotes reverence shown to a person in witness of his excellence. Now two things have to be considered with regard to man's honor. The first is that a man has not from himself the thing in which he excels, for this is, as it were, something Divine in him, wherefore on this count honor is due principally, not to him but to God. The second point that calls for observation is that the thing in which man excels is given to him by God, that he may profit others thereby: wherefore a man ought so far to be pleased that others bear witness to his excellence, as this enables him to profit others.
Now the desire of honor may be inordinate in three ways. First, when a man desires recognition of an excellence which he has not: this is to desire more than his share of honor. Secondly, when a man desires honor for himself without referring it to God. Thirdly, when a man's appetite rests in honor itself, without referring it to the profit of others. Since then ambition denotes inordinate desire of honor, it is evident that it is always a sin.
Reply to Objection 1. The desire for good should be regulated according to reason, and if it exceed this rule it will be sinful. On this way it is sinful to desire honor in disaccord with the order of reason. Now those are reproved who care not for honor in accordance with reason's dictate that they should avoid what is contrary to honor.
Reply to Objection 2. Honor is not the reward of virtue, as regards the virtuous man, in this sense that he should seek for it as his reward: since the reward he seeks is happiness, which is the end of virtue. But it is said to be the reward of virtue as regards others, who have nothing greater than honor whereby to reward the virtuous; which honor derives greatness from the very fact that it bears witness to virtue. Hence it is evident that it is not an adequate reward, as stated in Ethic. iv, 3.
Reply to Objection 3. Just as some are heartened to do good and disheartened from doing evil, by the desire of honor, if this be desired in due measure; so, if it be desired inordinately, it may become to man an occasion of doing many evil things, as when a man cares not by what means he obtains honor. Wherefore Sallust says (Catilin.) that "the good as well as the wicked covet honors for themselves, but the one," i.e. the good, "go about it in the right way," whereas "the other," i.e. the wicked, "through lack of the good arts, make use of deceit and falsehood." Yet they who, merely for the sake of honor, either do good or avoid evil, are not virtuous, according to the Philosopher (Ethic. iii, 8), where he says that they who do brave things for the sake of honor are not truly brave.
Source: Summa Theologica (New Advent)