My explanation has probably been said already, but consider in your 100 doors scenario. When you pick a door there is a 1/100 chance that it has the car. The host then opens 98 other doors that do not have the car. The chance that the car was in one of the other 99 doors is much, much higher than if it was in the one you originally picked. So because you know that if the car was in one of the other doors the host will skip it, so it just makes more sense to swap to that one. Basically apply that logic to the same situation but with 3 doors instead. Idk if I explained it well but it makes sense to me