An illustration that Jesus uses is one of pruning. Some have said pruning is a very painful process where the Lord cuts and slashes us through things like sickness, death, poverty, and other forms of tragedy so that eventually we will bear more fruit. This teaching not only promotes problems as being good, but necessary, if we want to bear more fruit.

That thinking is not consistent with the rest of God's Word.  The text in John 15:2 makes it very clear that the pruning that Jesus speaks of is done through the Word that He has spoken unto us.

Paul said in 2 Timothy 3:16-17 that God's Word was given to us "for doctrine, for reproof, for correction, and for instruction in righteousness: That the man of God may be perfect, thoroughly furnished unto all good works." That is God's method of pruning us, and He doesn't need the devil's help. His word will make us "perfect, thoroughly furnished unto all good works."

That is not to say that we cannot learn through tragedy: but God has a better way. If we mistakenly think that God is bringing tragedy into our lives to make us more fruitful, then we'll not resist the tragedies and they will not flee from us. 

All of us will learn by hard knocks, but the man who welcomes them with wide open arms will suffer greatly and be far behind the man who lets God's Word have His perfect work in him.