Where do lice come from? There is always someone who has lice in this world, for example, a homeless. He or she uses the same public transport as you do. And it might happen that some insects from his or her hair can get onto a seat, which you take later. As soon as a female louse gets onto your scalp, it starts oviposition. During one day it lays about 5 eggs, which transform to larvae in 7-10 days.
Among other causes of head lice is using borrowed things that are infested by lice. That is why you have to be very careful if you share your personal items with someone, but it is better not to land or borrow such things as combs, hair brushes, hats or anything else which may have contact with your hair.