In treating of the sins against charity, St. Thomas eventually treats of sloth, which is contrary to charity by being contrary to the first necessary effect of charity: joy. He says:
As Aristotle says in the Eighth book of the Ethics, no one can remain very long in sorrow (tristitia) without enjoyment. Two things must arise from sorrow: that man shun the sorrowful and that he go to something that gives him joy, just as those who cannot rejoice in spiritual enjoyments go to carnal ones
ut philosophus dicit, in VIII Ethic., nullus diu absque delectatione potest manere cum tristitia, necesse est quod ex tristitia aliquid dupliciter oriatur, uno modo, ut homo recedat a contristantibus; alio modo, ut ad alia transeat in quibus delectatur, sicut illi qui non possunt gaudere in spiritualibus delectationibus transferunt se ad corporales
Note how matter-of-factly St. Thomas speaks: two things must arise… just as those who cannot rejoice…
The Latin tristitia is broader than the English sorrow. St. Thomas speaks of rust being the tristitia of iron- it is the state of being run down and corrupted. In human beings it is depression and other like feelings.
No human mood is ever continuous, but some predominate. And if we don’t enjoy spiritual things we will just seek happiness in physical ones. St. Thomas speaks of enjoyment-seeking as pretty much automatic.
Kant’s ethics held up unpleasant action as the ideal of virtue. Those who forever “toughed it out” or followed sheer rational duty without enjoyment were most ideal. Isn’t there some plausibility to this? Virtue seems above all manifest in difficult and unpleasant circumstances, so why not think that continuous perfect virtue would be continuous unpleasant circumstances? St. Thomas appears to think such virtue is not only not ideal, but that it is simply impossible. The man who lived that way- taking no spiritual joy- would, in short order, be wallowing in purely physical pleasures.