But in order to acquire humility, it is necessary also to be prudent in not speaking well of oneself. "Let another praise thee," says the inspired word, "and not thy own mouth, a stranger and not thy own lips." [Prov. xxvii, 11]
It is very easy for us to fall into this fault of praising ourselves "Until it becomes a habit, and with this habit so opposed to humility how can we be humble?
What good qualities have we of our own for which we can praise ourselves? All the good that is in us comes from God, and to Him alone we must give praise and honor. When, therefore, we praise ourselves we are usurping glory which is due to God alone. Even though in praising ourselves we sometimes refer all to the honor of God, it matters little; when there is no absolute necessity it is better to abstain from self-praise, for although we refer all to the glory of God with our lips, our ingenious and subtle self-love cannot fail to appropriate it secretly. And even speaking depreciatingly of ourselves there may lurk some hypocritical pride in our words, such as was mentioned by the sage of old when he said: "There is one that humbleth himself wickedly, and his interior is full of deceit." [Ecclus xix, 23]
Therefore we can never watch over ourselves enough, because there is nothing that teaches us so well to know the pride of our heart as our words, with which we either reveal or hide the depravity of our affections. And this is the characteristic of the proud, according to St. Bernard: "One who boastfully proclaims what he is, or lies about what he is not." [Epist. lxxxvii]
Let us bear in heart and mind this precious advice given by Tobias to his son: "Never suffer pride to reign in thy mind or in thy words." [Tob. iv, 14] The words of a proud man are nauseous, whether he speaks of himself or others, and they are hated both by God and man: therefore we should detest this vice, not only from the Christian but also from the human standpoint."
-- Humility of Heart by Fr Cajetan Mary da Bergamo