To Tell a Lie

I've been thinking about lying. Why do people lie? Is it pride, intimidation, insecurity? Kids do it because they don't want to get in trouble, but why adults? We know better-right! I know we aren't perfect and that it happens, but I was recently in an interesting situation. I told a friend (friend A) something about another friend (friend B) in casual conversation. Not gossip just a response to a question asked by friend A about friend B. Then later (not in my presence) Friend B lied to Friend A and I looked like the liar! How funny is that! Any way I've decided to always tell the truth no matter how painful or embarrassing. It always turns out better that way. People just loose respect for you if you lie. In my case the true liar was revealed in the end, lucky for me!
But are there times when a lie is the best idea? What if someone asks you, something like, if you think they look fat, and you do think they look fat, what then? What if you know your friends husband is doing something terrible behind her/his back, what then? What if someone tells you something and tells you not to tell, no matter what, what then. What are the rules?
The rules are as follows:
1. If you are in contact with a person at least 5 days of the week you are allowed to tell them they look fat and work on a solution together. If not, it's not worth it, just lie.
2. Always tell the wife/husband what she/he doesn't know. Anonymously if you have to, like a letter.
3. Never tell someone something private! It will get out! Keeping secrets is self destructive, no one should be asked to do it. So don't tell until your ready for people to know about it!!!