Objection 1. It seems that prodigality is not a sin. For the Apostle says (1 Timothy 6:10): "Covetousness [Douay: 'desire of money'] is the root of all evils." But it is not the root of prodigality, since this is opposed to it. Therefore prodigality is not a sin.
Objection 2. Further, the Apostle says (1 Timothy 6:17-18): "Charge the rich of this world... to give easily, to communicate to others." Now this is especially what prodigal persons do. Therefore prodigality is not a sin.
Objection 3. Further, it belongs to prodigality to exceed in giving and to be deficient in solicitude about riches. But this is most becoming to the perfect, who fulfil the words of Our Lord (Matthew 6:34), "Be not... solicitous for tomorrow," and (Matthew 19:21), "Sell all [Vulgate: 'what'] thou hast, and give to the poor." Therefore prodigality is not a sin.
On the contrary, The prodigal son is held to blame for his prodigality.
I answer that, As stated above (Article 1), the opposition between prodigality and covetousness is one of excess and deficiency; either of which destroys the mean of virtue. Now a thing is vicious and sinful through corrupting the good of virtue. Hence it follows that prodigality is a sin.
Reply to Objection 1. Some expound this saying of the Apostle as referring, not to actual covetousness, but to a kind of habitual covetousness, which is the concupiscence of the "fomes" [Cf. I-II, 81, 3, ad 2], whence all sins arise. Others say that he is speaking of a general covetousness with regard to any kind of good: and in this sense also it is evident that prodigality arises from covetousness; since the prodigal seeks to acquire some temporal good inordinately, namely, to give pleasure to others, or at least to satisfy his own will in giving. But to one that reviews the passage correctly, it is evident that the Apostle is speaking literally of the desire of riches, for he had said previously (1 Timothy 6:9): "They that will become rich," etc. On this sense covetousness is said to be "the root of all evils," not that all evils always arise from covetousness, but because there is no evil that does not at some time arise from covetousness. Wherefore prodigality sometimes is born of covetousness, as when a man is prodigal in going to great expense in order to curry favor with certain persons from whom he may receive riches.
Reply to Objection 2. The Apostle bids the rich to be ready to give and communicate their riches, according as they ought. The prodigal does not do this: since, as the Philosopher remarks (Ethic. iv, 1), "his giving is neither good, nor for a good end, nor according as it ought to be. For sometimes they give much to those who ought to be poor, namely, to buffoons and flatterers, whereas to the good they give nothing."
Reply to Objection 3. The excess in prodigality consists chiefly, not in the total amount given, but in the amount over and above what ought to be given. Hence sometimes the liberal man gives more than the prodigal man, if it be necessary. Accordingly we must reply that those who give all their possessions with the intention of following Christ, and banish from their minds all solicitude for temporal things, are not prodigal but perfectly liberal.
Source: Summa Theologica (New Advent)