I is emphatic, implying that there are enough others who caused them trouble.
Who then is he, etc.
The thought underlying the passage, 1:24-2:3, is that the apostle's own joy is bound up with the spiritual prosperity of the Church. Compare Phi 4:1. As the helper of their joy he would receive joy through their faith and obedience. So long as their moral condition compelled him to come, bringing rebuke and pain, they could not be a source of joy to him. If I must needs make you sorry with merited rebuke, who can give me joy save you who are thus made sorry?