Finally got around to writing this post...
On Happy Catholic, Julie quotes St. Francis de Sales:
The devil takes advantage of sadness to tempt the good, striving to make
them sorrowful in their virtue as he strives to make the wicked rejoice
in their sins, and as he can only tempt us to evil by making it appear
attractive, so he can only tempt us away from what is good by making it
appear unattractive. He delights to see us sad and despondent because he
is such himself for all eternity and wishes everyone to be as he is.
St. Francis is talking about sloth or acedia, which isn't mere laziness but a kind of sorrow over a good. St. Gregory the Great wrote,
"From melancholy there arise malice, rancour, cowardice, despair,
slothfulness in fulfilling the commands, and a wandering of the mind on
unlawful objects." Not a good thing at all.
St. Thomas distinguishes the capital vice of sloth from the generalized sorrow that accompanies each sinful act -- e.g., "the lustful man is sorrowful about the good of continence, and the glutton about the good of abstinence" -- this way:
...the sorrow whereby one is displeased at the spiritual good which is in each act of virtue, belongs, not to any special vice, but to every vice, but sorrow in the Divine good about which charity rejoices, belongs to a special vice, which is called sloth.
In other words, sloth is sadness over what makes the lover of God joyful. If in thinking about loving God your heart sinks instead of rises, then you are slothful.
...the movement of sloth is sometimes in the sensuality alone, by reason of the opposition of the flesh to the spirit...; whereas sometimes it reaches to the reason, which consents in the dislike, horror and detestation of the Divine good, on account of the flesh utterly prevailing over the spirit.On this case it is evident that sloth is a mortal sin.
If a mortal sin is a sin that destroys charity, which is the love of God in one's soul, then yes, detestation of the Divine good is a mortal sin.
That's not to be glib; obviously rejoicing in something prevents you from sorrowing in that same thing. But if you believe that rejoicing in the LORD must be your strength, then you'll put in the work needed to rejoice in the LORD. That work might be as simple as praying, "Lord, make me rejoice in You," throughout the day. Or maybe there's some studying to be done, to better understand what it is about God that produces joy in the heart of the one who loves Him, or what it is about loving God that is attractive (the world, the flesh, and the devil have pretty much filled us in on the unattractive parts).
Any bad habit -- of sloth, say, or of malice, rancor, cowardice, despair,
slothfulness in fulfilling the commands, or a wandering of the mind on
unlawful objects -- is best uprooted by planting a good habit in its place, and planting a new habit takes time and discipline. Really, though, isn't the habit of joy worth a little time and discipline?