If someone does something good, let them repeat it; let them desire it; accumulating good produces happiness.

Direct contrast to the previous verse (117). Where 117 spoke of not repeating evil and not desiring it, 118 speaks of repeating good and cultivating the desire to do it.

Kayirāth’ enaṃ punappunaṃ — let them repeat it: the repetition that was dangerous in the context of evil becomes precious in the context of good. Repetition consolidates virtuous habits.

Tamhi chandaṃ kayirātha — let them desire it: cultivating chanda (intention, desire, aspiration) toward good is an active practice. Not waiting to feel the desire naturally but cultivating it deliberately through meditation, reflection and noble company.

Sukho puññassa uccayo — accumulating good produces happiness: the same mechanism of compound interest but in positive direction. Accumulated good actions create an inertia of wellbeing that feeds and expands naturally.