John Gill Commentary Galatians 4:18

"But it is good to be zealously sought in a good matter at all times, and not only when I am present with you." — Galatians 4:18 (ASV)

But it is good to be zealously affected
A zealous affection when right is very commendable, as the instances of Phinehas, Elijah, John the Baptist, and our Lord Jesus Christ show, and a contrary spirit is very disagreeable. But then it must be expressed

in a good thing ;
in a good cause, for God, and the things of Christ; for the Gospel, and the ordinances of it, and for the discipline of God's house, and against immorality and profaneness, errors and heresies: and it should be "always"; not at certain times, and upon some particular accounts, but it should be constant, and always continue; it should be ever the same towards God, Christ, and his ministers:

and not only when I am present with you ;
by which the apostle suggests, that while he was with them they were zealously attached to him and truth; but no sooner was he gone from them, but their zealous affection abated, and was fixed on others, which discovered their weakness, fickleness, and inconstancy; whereas he was always the same to them, and bore the same love to them, as the following words show.