'It makes no sense to me,' said Waylander, 'but then life itself makes no sense. So it becomes reasonable.'

Waylander by David Gemmell

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From the Discworld series by Terry Pratchett (not one liners, but still the best quotes in fantasy IMO. There are hundreds, I'm picking a few I really like):

… and then Jack chopped down the beanstalk, adding murder and ecological vandalism to the theft, enticement and trespass charges already mentioned, but he got away with it and lived happily ever after without so much as a guilty twinge about what he had done. Which proves that you can be excused just about anything if you’re a hero, because no one asks inconvenient questions.

Hogfather

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Death was standing behind a lectern, poring over a map.

YOU HAVEN’T HEARD OF THE BAY OF MANTE, HAVE YOU? he said.

‘No, sir,’ said Mort.

FAMOUS SHIPWRECK THERE.

‘Was there?’

THERE WILL BE, said Death, IF I CAN FIND THE DAMN PLACE.

Mort

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The wizards were civilized men of considerable education and culture. When faced with being inadvertently marooned on a desert island they understood immediately that the first thing to do was place the blame.

The Last Continent

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The best one:

‘All right,’ said Susan. ‘I’m not stupid. You’re saying humans need … fantasies to make life bearable.’

REALLY? AS IF IT WAS SOME KIND OF PINK PILL? NO. HUMANS NEED FANTASY TO BE HUMAN. TO BE THE PLACE WHERE THE FALLING ANGEL MEETS THE RISING APE.

‘Tooth fairies? Hogfathers? Little—’

YES. AS PRACTICE. YOU HAVE TO START OUT LEARNING TO BELIEVE THE LITTLE LIES.

‘So we can believe the big ones?’

YES. JUSTICE. MERCY. DUTY. THAT SORT OF THING.

‘They’re not the same at all!’

YOU THINK SO? THEN TAKE THE UNIVERSE AND GRIND IT DOWN TO THE FINEST POWDER AND SIEVE IT THROUGH THE FINEST SIEVE AND THEN SHOW ME ONE ATOM OF JUSTICE, ONE MOLECULE OF MERCY. AND YET— Death waved a hand. AND YET YOU ACT AS IF THERE IS SOME IDEAL ORDER IN THE WORLD, AS IF THERE IS SOME … SOME RIGHTNESS IN THE UNIVERSE BY WHICH IT MAY BE JUDGED.

Yes, but people have got to believe that, or what’s the point—’

MY POINT EXACTLY.

Hogfather

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Edit: one more:

“Insofar as I may be heard by anything, which may or may not care what I say, I ask, if it matters, that you be forgiven for anything you may have done or failed to do which requires forgiveness. Conversely, if not forgiveness but something else may be required to insure any possible benefit for which you may be eligible after the destruction of your body, I ask that this, whatever it may be, be granted or withheld, as the case may be, in such a manner as to insure your receiving said benefit. I ask this in my capacity as your elected intermediary between yourself and that which may not be yourself, but which may have an interest in the matter of your receiving as much as it is possible for you to receive of this thing, and which may in some way be influenced by this ceremony. Amen.”

Mardak's prayer

Creatures of Light and Darkness by Roger Zelazny