…sloth, as we understand it here, denotes sorrow for spiritual good…
Summa Theologica, II-II Q 35, A1
Our cultural attitude towards the sin of sloth is incorrect. There tends to be the notion that a person who is slothful is by definition also lazy. There is the possibility that laziness is a manifestation of sloth as an external dimension to this sin. However, laziness can actually be virtuous in the sense of rest and avoiding work. Again, we are speaking of a spiritual sin, not a sin of the flesh. That means that something within the intellect of the person is gone askew. St. Thomas explains that there is a type of sadness at the spiritual good. Later he calls it the Divine Good - namely God Himself, makes us sad.
The particular example he gives us in regard to sloth is a sadness at the prospect of resting on Sunday, during the mass. This sadness is so strong that it oppresses us, and prevents us from acting zealously about God.
The reason this is incredibly important to consider is due to the idolatry of work in our current culture. Rest is perceived to be something we ought to feel guilty for. Rest after all, according to the culture, is only present in our life to give us the energy to work further. In reality, the Church teaches that anthropologically, man was created to rest, and work is meant to order and facilitate such rest. That is to say, the exact opposite is true.
So if we live in a culture that considers make-work projects an idol, then sloth needs to become a vice that is defined by laziness rather than rest. The reality, however, is that we were created to rest in the Divine Good. To delight in God’s goodness, and to allow his love to penetrate the places of shame (through confession, counselling, prayer, friendship, etc.), to be swept up in His own Divine Mystery and sigh. The culture would look at these activities of the contemplative as utterly useless and the “waste of a life” or a “waste of time.” The Church would agree in part and say, it is useless, and for that reason, it is worthy.
You see, as humans we are not meant to be mere tools used for the purposes of whatever. Rather we are meant to playfully rest in God - most especially in the next life, and with glimpses of such rest in this life. Sunday worship, where we commune with Christ who is our Sabbath (rest), is the pivotal place where the soul is meant to leap with joy, not sorrow. We do not attend to mass, or any spiritual activity (God), as though He is the burden.
Yet as fallen human beings, the shame that causes us to hide from God causes us to prefer the toil and labor of the desert to the original purposes God designed within us. We escape into work in order to avoid the deeper contemplation of things. And if we meditate, it is a vacuous type of meditation that is merely emptiness, rather than the fullness of God.
The counter virtue to sloth is zeal. But zeal is not merely a passion, it is also a calm and peaceful commitment.
Check out this brief video on the sin of Sloth
…the proper effect of charity is joy in God, as stated above (II-II:28:1), while sloth is sorrow about spiritual good in as much as it is a Divine good. Therefore sloth is a mortal sin in respect of its genus…
Summa Theologica, II-II Q. 35, A3