People lie. A lot. For a lot of different reasons.

"I don't really respect you as a person but I don't want you to be angry at me and I don't want to feel like a bad person so I'm going to lie."

"The truth is private or embarrassing so I'm going to lie."

"You are a small child and you'll be happier if I tell you this so I'm going to lie."

"I would like to make myself look more intelligent/cooler/nicer/better-in-any-way-possible so I'm going to lie."

"I don't want to see you get hurt by what I did so I'm going to lie to you."

See, thing is, the last one is just the first one lying to you so it looks nicer. Lying is manipulation, of your thoughts, your feelings, the truth, your perception of them. It's trying to control the way people see things. Essentially crippling their ability to make a decision. If you lie to someone so they will keep being your friend you have just tricked them into being your friend.

On the other hand there's lying about your opinion. This is debatable.

If you are at an event and someone asks you your opinion on their clothes, you probably say something positive. You have just boosted their self confidence, and after all it is their shirt, if they like it and feel happy in it that's what matters. Then again if you are lying to them and it's a butt-ugly shirt they now think it's a nice shirt, continue to wear it in public and think they look good while lowering the opinion of those around them.

The 'it's okay to lie about this thing' logic leads to bigger lies. As long as the lie makes other people happy we think it's okay to tell it. A man is cheating on his wife, well she's happier not knowing isn't she? Soon everyone's happiness is based on lies. We have an entire society based on lies. Because it doesn't matter who you use, hurt manipulate, as long as someone comes out of the situation happy. People do it all the time. It's just a little white lie.