St. John Cassian: Hence sadness is to be judged beneficial for us...

'Hence sadness is to be judged beneficial for us in one instance alone -- when we conceive it out of repentance for our sins and are inflamed by a desire for perfection, and by the contemplation of future blessedness. 

Of this the blessed Apostle himself says: "The sadness that is in accordance with God works repentance unto a lasting salvation, but the world's sadness works death."
[II Corinthians 7:10]'

St. John Cassian