An excerpt from The Prophet by Kahlil Gibran. 

...a woman said "Speak to us of Joy and Sorrow."

And he answered:

Your joy is your sorrow unmasked.

And the same well from which your laughter rises

 was oftentimes filled with your tears.

And how else can it be?

The deeper that sorrow carves into your being,

 the more joy you can contain.

When you are joyous, look deep into your heart and you shall find

 it is only that which has given you sorrow that is giving you joy.

When you are sorrowful look again in your heart, and you shall see

 that in truth you are weeping for that which has been your delight.

Some of you say, "Joy is greater than sorrow,"

 and others say, "Nay, sorrow is the greater."

But I say unto you, they are inseparable.

Together they come, and when one sits alone with you at your board,

 remember that the other is asleep upon your bed.

Only when you are empty are you at standstill and balanced