Jamieson, Fausset and Brown on 1 Thessalonians 3:10

10. Night and day--(See on 1Th 2:9). Night is the season for the saint's holiest meditations and prayers (2Ti 1:3).

praying--connected with, "we joy"; we joy while we pray; or else as Alford, What thanks can we render to God while we pray? The Greek implies a beseeching request.

exceedingly--literally, "more than exceeding abundantly" (compare Ep 3:20).

that which is lacking--Even the Thessalonians had points in which they needed improvement [Bengel], (Lu 17:5). Their doctrinal views as to the nearness of Christ's coming, and as to the state of those who had fallen asleep, and their practice in some points, needed correction (1Th 4:1-9). Paul's method was to begin by commending what was praiseworthy, and then to correct what was amiss; a good pattern to all admonishers of others.