To me, I figure that a trustworthy person is one who will disclose the information to you, because the people who deserve to be trusted are the people who are open about their motives.
I suppose there'd be an exception if there were five seconds before you would mysteriously die and someone asked you to trust them to do something risky to survive.
Other than that, I need to have as many facts as are available so I can evaluate the plan/situation in order to determine what the most logical decision is. Besides, people are unpredictable, so even a really nice person who you think you like or love could end up taking advantage of you...or killing you. Either way.
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"There are things you need not know of, though you live and die in vain,
There are souls more sick of pleasure than you are sick of pain"
--G. K. Chesterton, The Aristocrat