A quote from "The Prophet", Kahlil Gibran.
A favourite book of mine that has been uncracked for far too long.
The full passage is:
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.