8 great reasons to tell the truth~~Barbra A. Lewis 

1. Lets everyone know what really happened. There's less chance of misunderstanding confusion or conflict
2. protects the innocent people from being blamed or punished.
3. Allows everyone to learn from what happened
4. You usually get into less trouble for telling the truth other than for lying and getting caught
5. Other people trust you more when you tell the truth 
6. you dont have to tell  and remember more lies to keep your story straight
7. Gain a reputation for being truthful - a trait most people value
8. helps you feel secure and peaceful inside.

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Well... these are good reasons/rules to abide by. But its not always good to tell the truth. Sometimes its better to tell a lie, ie when people are scared or distressed. You dont want to make them more scared by telling them the truth, you wanna try to ease their nerves. Also some people rather here a sweet lie than a tough truth.So i guess it all depends on your situation.