For when many rejoice together, the joy of each one is richer: they warm themselves at each other’s flame.

[Quando enim cum multis gaudetur, et in singulis uberius est gaudium, quia fervefaciunt se et inflammantur ex alterutro.]

Augustine of Hippo (354-430) Christian church father, philosopher, saint [b. Aurelius Augustinus]

Confessions, Book 8, ch. 4 / ¶ 9 (8.4.9) [tr. Sheed (1943)] (c. AD 398)
    (Source)

(Source (Latin)). Alternate translations:

For when many joy together, each also has more exuberant joy for that they are kindled and inflamed one by the other.
[tr. Pusey (1838)]

For when many rejoice together, each also has more exuberant joy; for that they are kindled and inflamed one by the other.
[ed. Shedd (1860)]

For when many rejoice together, the joy of each one is the fuller in that they are incited and inflamed by one another.
[tr. Pilkington (1876)]

For when many rejoice together, in each there is an overflowing joy, for they kindle themselves and are kindled by one another.
[tr. Hutchings (1890)]

For, when joy is shared with many, the joy of each is richer, because they warm one another, catch fire from one another.
[tr. Bigg (1897)]

For when many rejoice together the joy of each one is fuller, in that they warm one another, catch fire from each other.
[tr. Outler (1955)]

For when many men rejoice together, there is a richer joy in each individual, since they enkindle themselves and they inflame one another.
[tr. Ryan (1960)]

When large numbers of people share their joy in common, the happiness of each is greater because each adds fuel to the other’s flame.
[tr. Pine-Coffin (1961)]

For when many people rejoice together, the joy of each individual is all the richer, since each one inflames the other and the warmth spreads throughout them all.
[tr. Warner (1963)]

For when joy is shared with many, joy is fuller in each. They grow ardent and are fired each by the other.
[tr. Blaiklock (1983)]