Your pain is the breaking of the shell that encloses
your understanding.
Even as the stone of the fruit must break, that its
heart may stand in the sun, so must you know pain.
And could you keep your heart in wonder at the
daily miracles of your life, your pain would not seem
less wondrous than your joy;
And you would accept the seasons of your heart,
even as you have always accepted the seasons that
pass over your fields.
And you would watch with serenity through the
winters of your grief.
Much of your pain is self-chosen.
These lines are instantly evocative of the book of  Kohelet  (Ecclesiastes) in the Hebrew Bible. Ecclesiastes is a book of wisdom written by  King Solomon  in his old age (he wrote The Song of Songs in his youth, and Proverbs while middle age). In  the book, we encounter this wisdom: 
 To every thing there is a season, and a time to every purpose under the heaven: A time to be born, and a time to die; A time to plant, and a time to pluck up that which is planted; A time to kill, and a time to heal; A time to break down, and a time to build up; A time to weep, and a time to laugh; A time to mourn, and a time to dance; A time to cast away stones, and a time to gather stones together; A time to embrace, and a time to refrain from embracing; A time to get, and a time to lose; A time to keep, and a time to cast away; A time to rend, and a time to sew; A time to keep silence, and a time to speak; A time to love, and a time to hate; A time of war, and a time of peace. 
 
 What Solomon was saying was that everything has a place and a purpose in life: he draws a parallel between the seasons of nature (a time to plant, and a time to pluck up that which is planted) and the seasons of the heart (a time to mourn, and a time to dance). 
 Kahlil extends this tradition in this line:  as you accept the agricultural seasons without questions, and you accept the cycle of nature (the tides, the wax and wane of the moon, etc.), so you should consider your own fluctuations through emotion natural.
On Pain