Variant: Someone I loved once gave me a box full of darkness. It took me years to understand that this too, was a gift.
Source: Thirst
Related quotes

“Boxing is my life, but my life is not only boxing.”
Source: [Афоризми відомих українців, Folio, 2009, 978-966-03-4817-2, Kharkiv, 86, uk]

“A happy life consists not in the absence, but in the mastery of hardships.”
The Simplest Way to be Happy (1933)

“Sweet spring, full of sweet days and roses,
A box where sweets compacted lie.”
Virtue, reported in Bartlett's Familiar Quotations, 10th ed. (1919)

address to LULAC (July 1, 2005)
2007, 2008

In Legacy of Two Rich Voices - Upholding tradition without being traditionalists http://www.telegraphindia.com/1100425/jsp/opinion/story_12359659.jsp