Philippians 2:27 KJV
For indeed he was sick nigh unto death: but God had mercy on him; and not on him only, but on me also, lest I should have sorrow upon sorrow.
The book of Philippians is a book of joy and rejoicing. This is where we hear "Rejoice in the Lord alway and again I say rejoice." 
This is where we hear, "Be careful for nothing but in everything by prayer and supplication with thanksgiving let your requests be made known unto God." 
But Philippians does not deny the natural sorrows of life when suffering.
The Apostle was suffering.
- He was in prison
- He had lost a good deal of companionship
- He had seen Demand forsake him and
- He had grieved that Epaphroditus was sick
God's mercy had been shown to Epaphroditus in healing him but also to Paul. He confesses his sorrow would have been compounded by the loss.
Sorrow is normal and natural. It is a part of what it means to be human. It is right and proper and healthy to sorrow. But it should never stifle us. God's mercy is always available and God's grace is always sufficient.
Sorrow when appropriate. But in sorrow seek the Lord. Find his grace for strength and recover joy.