(38) Not to be slothful (cf Rom 12:11).
The scripture passage that St. Benedict quotes from St. Paul's Letter to the Romans. I expected it to be the passage "if a man doesn't work, he shouldn't eat," but its not that. The passage he quotes is "Never flag in zeal, be aglow with the Spirit, serve the Lord."
Again, like in all previous counsels the motivation to not be slothful is to be animated by God's Holy Spirit. How do we receive this Spirit, by serving the Lord (I like to think of this as "working for the Lord").
Just as one might take a job with a certain company and enjoy certain benefits that the company offers, so too for the person who "works" for the Lord. The chief benefit that God provides to those who serve Him is that He gives them the power to fulfill the job. He also fills His workers with the desire and zeal to do the work.