Here's what John doesn't yet understand.    Let me try this again, if not for John, then for others who might be curious what the Catholic Church teaches about suffering, pain, and intimate union with God (see Salvivici Doloris for some very beautiful writing & teaching on this matter). 

St Paul is talking about deep & intimate unity with Jesus Christ, in fact a total and trusting abandonment into His care, and a oneness with the will of God.  His motives are our motives.

St Paul is "rejoicing" in the sufferings that God is allowing for the good of others.

He is imitating Jesus Christ, even in suffering. 

One can't do that without a ready presence (influence) of God,  a lively interior life.

As I said, John doesn't yet have the integrality of Scripture needed to fit such matters together, how "happiness & pain" are not at odds.  

How one can actually rejoice while suffering. 

Jesus sang the Psalms on His way to Calvary.  So can we, so should we if we want an ever closer union.