That’s one of the many basic lessons Ms Ellen Chisa learned while in business school (read her entire essay here):

“There were a couple distinctly different attitudes in the [classroom]. I genuinely try to give advice that helps the person who asks me, removing my personal interest. I try to assume the best of others, and thus think they do similarly for me. While my belief was prevalent in the room, there was also a counter view of ‘the world works best if you serve your own interest and everyone should believe that others are self-serving.’ ”

How about a third view: trust no one–until the would-be “trustee” has done something to earn your trust? (File this entry under “costly signalling vs. cheap talk.” Via Marginal Revolution.)