“You're always the hero when you can pick and choose your memories. You're always the villian when you remember everything.”
When you remember everything you not only remember what you did right, you also remember your mistakes great and small. Also others seldom like to be made aware of their own mistakes that you may remember as well. They tend to get upset when reminded of them.
Related quotes

as quoted by [James W. Botkin, Dan Dimancescu, Ray Stata, The innovators: rediscovering America's creative energy, Harper & Row, 1984, 0060152850, 165]

Daybreak http://books.google.com/books?id=Imte1JcsQ64C&q=%22You+don't+get+to+choose+how+you're+going+to+die+Or+when+You+can+only+decide+how+you're+going+to+live+Now%22&pg=PA135#v=onepage (1968)
Variant or paraphrase: You can't decide how you're going to die. Or when. What you can decide is how you're going to live now.

Quotes from Judge Judy cases, Dress, stand, speak properly
Source: http://www.youtube.com/user/JJMinisodes#p/u/7/hpLSM73I6ZM ("If you tell the truth, you don't have to remember anything" - Mark Twain)

Source: Dandelion Wine (1957), p. 142
Context: “I don’t know,” he admitted.
“Well.” She started pouring tea. “To start things off, what do you think of the world?”
“I don’t know anything.”
“The beginning of wisdom, as they say. When you’re seventeen you know everything. When you’re twenty-seven if you still know everything you’re still seventeen.”
“You seem to have learned quite a lot over the years.”
“It is the privilege of old people to seem to know everything. But it’s an act and a mask, like every other act and mask. Between ourselves, we old ones wink at each other and smile, saying, How do you like my mask, my act, my certainty? Isn’t life a play? Don’t I play it well?”
They both laughed quietly.

“You never know when you're making a memory”