@vonny,
Often I'm not sure other people want to hear the truth, they're seeking agreement, or support, or encouragement, and truthful responses might only lead to hurt feelings or conflicts. If we were totally truthful, all of the time, with everyone, I think it would be difficult to maintain our relationships or social functioning on an even keel.
We learn to be tactful, or diplomatic, which are often ways of dancing around the truth, or just trying to keep the peace, or of avoiding hurting someone's feelings, or avoiding an argument. Good social skills are often at odds with being totally honest.
If someone gives me a gift I really don't care for, but they've put effort and expense into getting it for me, should I really tell them I think they have lousy taste and, if I can't exchange it, I'll shove it in a drawer or closet so I don't have to even look at it? Or if an acquaintance invites me for dinner, and asks if I like what they've prepared, should I make negative comments about the food, if that's how I honestly feel about it? And, many people who are totally honest with a boss, might wind up losing their jobs.
I really try to avoid being dishonest, but I don't find it possible, or even desirable, to be totally honest, all of the time, with everyone.