Truth is a Wonderful Thing
Except when it's not. If I told the truth (or expressed what I was thinking) all the time, my family, friends, and colleagues would not talk to me. In fact, some might be shocked to hear my true views on things, especially their behaviors.
Everyone has flaws. Most can be overlooked or merely accommodated. Some, however, get worse over time, and it can affect interactions. A lot of times, someone does something that gets you irritated, but you decide not to say anything. It happens again and again, so you decide to say something. The reaction, which is rarely "thank you for pointing that out," is normally anger, defensiveness, and conflict. Moreover, the person believes that your failure to say anything earlier was your acceptance that there was nothing wrong.
Only the most incredible people want to hear criticism so they can do better. Most of us prefer thinking that we are doing a great job, and the other guy who criticizes is being petty. True, criticism makes one feel uncomfortable, but would you rather have resentment build up or bad habits become a core personality trait? Behavior modification will not take place if the person is not aware of the problem, but making him aware of the problem creates problems. Truth is the problem; deceit avoids conflict. "No, it's all right. I don't mind." Bullshit.
When the people you love need to hear the truth, it becomes even more uncomfortable. Frankly, I don't care if the driver who doesn't use turn signals overhears that I think he's a worthless piece of crap. On the other hand, I don't want my wife to get angry or hurt because I told her it bothers me that she moves my stuff, all the time, without asking. As Nicholson said in A Few Good Men: "You don't want to know the truth."
Instead, we become passive aggressive, employ mild (or wild) sarcasm, roll our eyes, or hope that person notices our subtle disapproving reaction. That rarely works, and sometimes the resentment grows. In the case of people you love and/or respect, the best thing to do is try to be a better person yourself. If your behavior conforms to what you expect from them, they just might get the hint. In a pinch, you might also consider acting hurt yourself and make them ask you why. It might soften the blow of you saying: "You've been acting like a jerk."
With strangers, the benefit of truth telling is directly proportional to their ability to hurt you or embarrass you. Obviously, if they have a gun, you might want to be more discreet. Saying unkind, but truthful, things to a child, old person, or disabled individual will frequently make you look much worse than just keeping your mouth shut. As someone, maybe Mark Twain said: "Better to say nothing and be thought a fool than opening your mouth and removing all doubt."
Tell the truth, except when you can't.