Charles Spurgeon once wrote to his students in the book, “Lectures to my Students”…  

"Fits of depression come over most of us.  Usually cheerful as we may be, we must at intervals be cast down.  The strong are not always vigorous, the wise not always ready, the brave not always courageous, and the joyous not always happy.   There may be here and there men of iron…but surely the rust frets even these.”

4:13