Telling the truth is the foundation of any kind of relationship even though is Laboral, academic or with family members. Telling the truth has many advantages not only for the person who received the information but also for who said it. The followings are the main reasons why  I believe people must say always the truth. Telling the truth makes people free. Even it is complicated or even people can feel uncomfortable saying it at first. Being honest can give people the option to do what is good for them. When people say openly their falts or their mistakes, they do not have to worry more about it. They have realized themselves from problems. They had freed themselves of negative consequences or bad actions. For example, I kid broke an expensive plate and later on,  his mom finds out about this.  The mother may ask who did it, and the kid bravely makes responsible for his act. The kid may be punished by his mother, however, he assumed his part, his responsibility. He did not have to tell lies, like saying that it was his brother's falt. He frees himself of bad actions.  Telling the truth make people more conscious of their limitations. When people tell the truth to others, they are also telling the truth to themselves. Telling lies sometimes could be seen as positive in the short term. For example, a scholar can make impressive research using the ingenue of another person for his benefits. At first glaze, he may receive awards and recognition of the academic circle. But his lie can be uncovered at any time. He may be worried about this. In addition, because he has not been authentic with others and himself, he would never know his limitations and his achievements. Those will be shadowed by the efforts of another person.
Finally, telling the truth can improve and maintain good relationships with others. People become worthed and honorable, and people will start looking more to them for any kind of favor or service. This is a valuable quality in the business world. For example, a businessman is selling a product. This product is not so good as the sellers say it is. People will find out this, and they will start feeling angry with the businessmen for being telling them lies.  Consumers are not going to contact the seller anymore because they don't see the seller as trustworthy. Finally, the businessman will be struggling in keeping his business afloat.