If you're not inserting a bit of tension into your relationships from time to time, you're probably getting screwed.
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Oops, I just realized that I got your tweet associated with the wrong thing. If you are buying something from a vendor, you need to bust their balls on price and threaten to go elsewhere every once in a while. If you don't do that, they're going to take you for a ride.
Applies to everything, not just vendors. If you're a manager and you're afraid to give negative feedback, you're going to be very ineffective.