I would encourage you to read the entirety of his Question on Fasting (ST IIa.IIae, Q.147). All the following excerpts are taken from this one question, and no further citations will be given. Minor spacing changes have been made to the text to make it easier to read and understand. My own thoughts and emphasis.
St. Thomas on Fasting:
First, in order to bridle the lusts of the flesh, wherefore the Apostle says (2 Corinthians 6:5-6): "In fasting, in chastity," since fasting is the guardian of chastity. For, according to Jerome [Contra Jov. ii.] "Venus is cold when Ceres and Bacchus are not there," that is to say, lust is cooled by abstinence in meat and drink.
Secondly, we have recourse to fasting in order that the mind may arise more freely to the contemplation of heavenly things: hence it is related (Daniel 10) of Daniel that he received a revelation from God after fasting for three weeks.
Thirdly, in order to satisfy for sins: wherefore it is written (Joel 2:12): "Be converted to Me with all your heart, in fasting and in weeping and in mourning."Immediately upon reflecting on the order in which these purposes are given it seems confusing that the first and the third purposes are separated by the second. Upon closer examination, however, we can recognize in this logic that these three purposes can be boiled down to two ends. In 'bridling the lusts of the flesh' we avoid sin so as to 'order the mind to heavenly things.' So, even though the first and third share the same object, namely sin, the first actually shares the same end with the second purpose, namely turning the mind away from earthly things and toward heavenly.
The same is declared by Augustine in a sermon (De orat. et Jejun. [Serm. lxxii (ccxxx, de Tempore)]): "Fasting cleanses the soul, raises the mind, subjects one's flesh to the spirit, renders the heart contrite and humble, scatters the clouds of concupiscence, quenches the fire of lust, kindles the true light of chastity."The relationship between fasting and chastity may not be immediately obvious. When we consider the two ends of fasting, the relationship seems a bit clearer
Fasting is directed to two things, the deletion of sin, and the raising of the mind to heavenly things.For St. Thomas there are two acts of virtue that pertain to purity, namely chastity and continence. By chastity one withdraws from unlawful desires, continence from lawful desires which are lesser goods for the sake of giving full attention to higher goods. So, the abstinence and fasting of these good things is a practice in continence, for the sake of refraining from and standing firm against unlawful things including but not limited to lust. Since, however, lust is the strongest of our concupiscent passions, curbing it is difficult. Only by smaller acts of temperance can we strengthen our mind and body to turn toward God for the strength to avoid temptation and stand firm in the face of it.
Now, since fasting is directed to the deletion of sin, and the raising of the mind to heavenly things, St Thomas continues:
...fasting ought to be appointed specially for those times, when it behooves man to be cleansed from sin, and the minds of the faithful to be raised to God by devotion: and these things are particularly requisite before the feast of Easter, when sins are loosed by baptism, which is solemnly conferred on Easter-eve, on which day our Lord's burial is commemorated, because "we are buried together with Christ by baptism unto death" (Romans 6:4). Moreover at the Easter festival the mind of man ought to be devoutly raised to the glory of eternity, which Christ restored by rising from the dead, and so the Church ordered a fast to be observed immediately before the Paschal feast.That being the case, we should be particularly sensitive to our own needs. We should reflect on our own failures and need for fasting. The Church requires, as a bare minimum, us to forego full meals. We can have two smaller meals, and one full meal, on days of fasting. The two smaller meals combined should not equal that of the larger meal.
If we look at the purpose of fasting as being a means to practice virtue through the denial of our bodily appetites so as to strengthen ourselves against sin, we should be striving to do the most that we can for the sake of our salvation. Let us face it, we have grown spiritually lazy when we are satisfied with the minimum.
Imagine for a moment an person who wants to build muscle that only lifts two-pound weights three times for five sets all because it is the bare minimum that a person in physical therapy should be doing to maintain muscle mass. Now, we may not be olympic weightlifters, but surely we can try to lift what we can.
We need to be realistic about what we can handle. If we cannot handle more than the bare minimum at the beginning of Lent, maybe we should incorporate smaller fasts into our lives for more days per week for the entirety of Lent. Now that I have said my piece about fasting, here is what St. Thomas says about not fasting:
The "fasting of joy" proceeds from the instigation of the Holy Ghost Who is the Spirit of liberty, wherefore this fasting should not be a matter of precept. Accordingly the fasts appointed by the commandment of the Church are rather "fasts of sorrow" which are inconsistent with days of joy. For this reason fasting is not ordered by the Church during the whole of the Paschal season, nor on Sundays: and if anyone were to fast at these times in contradiction to the custom of Christian people, which as Augustine declares (Ep. xxxvi) "is to be considered as law," or even through some erroneous opinion... he would not be free from sin. Nevertheless fasting considered in itself is commendable at all times; thus Jerome wrote (Ad Lucin., Ep. lxxi): "Would that we might fast always."In short, fasting for the right reason is always profitable. Make the most of your Lent. Do not let this one go by without growing in virtue. And enjoy Sundays... temperately.