Why people must be honest?

Well, the website pushed me to write this entry, while I have no idea at all what to write. But, okay let's be write about some general topic "why people must be honest?"

Aristotle said that humans are Zoon Politicon, that means a social creature, which is people need people, and communities need other communities to continue the life. This situation creates interaction, and when there is dishonesty, the relationship is crushed. And that is my simple question, why must people be honest? Honesty doesn't crush a relationship. 

<em>I think it's because you can't trust a dishonest person, but he/she could hurt you or put you in danger.</em>