The Joker threatened Rachel Dawes at Harvey Dent's fundraiser at knifepoint. After he asked if she was troubled by his scars, Joker rhetorically asked if Rachel wanted to know how he received them. The Joker then related the following story, either an historical anecdote, or merely a tall tale, or possibly a combination of the two:
The man who later became the Joker had a young wife who was a gambler: She borrowed from loan sharks, and, after she was being unable to repay them due to her lost money, they cut her face, which resulted in disfiguring scars. Some time later, "Joker" then placed a razor blade on his cheeks and created an extension of a smile, known popularly as a Glasgow Smile or a Chelsea Grin. Joker then informed his wife that he did it in order to show her that her disfigured appearance did not bother him and that he still loved her.
Tragically, Joker's wife could not stand his newly disfigured face, which was likely in equal approximation to her own, left him, and never returned. That incident may have built upon "Joker's" views of the world around him, if it was not the major beginning factor. Whether or not that incident or his wife as Joker described her were real, remained unverifiable as the Joker was prone to relating conflicting anecdote's as to his facial scars' origin.
Behind the ScenesEdit
To enhance the popularity and mystery of the Joker's characters, his creators may never wished to reveal, or created, a true backstory that explained many aspects of his origins as his criminal reinvention as the Joker, including his cheek scars. They may or may not have been inflicted in the incident(s) that didn't kill him, but "made him stranger".
Coincidentally, the Joker in The Killing Joke also had a wife (Jeannie in that case). She also played a role in the Joker's fall into evil (in that case, due to dying in an accident that was related to a baby bottle heater) and there were some hints that she may not have actually existed.