Lying is a part of "social skills". It is not a necessary part -- you can usually avoid expressing opinions on topics where speaking truth would harm your image -- but it helps to make people feel better and do what you want.

I suppose most people are lying without being aware that they are lying; compartmentalization probably helps a lot. (But because I am not a good liar, you should not trust this hypothesis -- perhaps I am missing something very important here.)

I don't know what would the optimal amount of lying. It's probably context-dependent.