Some people habitually lie. The first lie is always the hardest but once you get past it and you go for the next and keep going without guilt, it becomes a habit and you just lie about everything. Once a liar always a liar. They will lie about how much sugar is in a recipe, how much time it took to drive somewhere, how much something cost, how much they ate, how old they are. It doesn’t matter the subject, they wouldn’t know the truth if it stared them in the face.
Any sin which is sometimes permitted will be abused.
Lying is one of them. One can change the truth for the sake of peace. Well, once you go down that slippery slope, you can fall all the way down.
One may get involved in Machloket for the sake of Hashem, which may include speaking L”H. The problem is, again, that once there is permission in a limited way, it doesn’t always stop there. (heard from Rav Frand)
People generally permit that which they themselves do.
These include small lies, small theft (like not paying tax), small bits of lashon hara (I mean, its not really LH right? I can probably try to trick myself into believing that)