The bud may have a bitter taste

Matthew 20:22
New King James Version (NKJV)
22 But Jesus answered and said, “You do not know what you ask. Are you able to drink the cup that I am about to drink, and be baptized with the baptism that I am baptized with?”[a] They said to Him, “We are able.”

William Cowper, an intimate friend of John Newton.
Cowper said, "A sincerer or more affectionate friend no man ever had"

Suffering from bouts of depression.
A sanctification in which God has ordained it for him.

In the sovereignty of God.
He has allow William Cowper to go through bouts of depression for reasons we struggle to comprehend.

But we know no one is capable to write beautiful poems like the way he do.
No one but Cowper is able to compose beautiful hymn that is uniquely his.

In a mysterious way that God has work out his life, such that no one but Cowper is able to glorify God the way God intended it to be.

~William Cowper
Judge not the Lord by feeble sense.
But trust Him for His grace;

Behind a frowning providence
He hides a smiling face.

His purposes will ripen fast,
Unfolding every hour;

The bud may have a bitter taste,
But sweet will be the flower.

Blind unbelief is sure to err
And scan His work in vain;

God is His own Interpreter,
And He will make it plain.

Reference:
~God Moves in a Mysterious Way
https://www.youtube.com/watch?v=6z0e_xSC8to&list=RD6z0e_xSC8to#t=12

~Reflections on the Life of William Cowper
http://www.desiringgod.org/messages/insanity-and-spiritual-songs-in-the-soul-of-a-saint