I don't know if it's because people learn from their parents, and their parents do it all the time, but drivers around here have this annoying habit of driving with their left foot resting on the brake pedal. I once asked someone I know who drove like this and they said it was because it was safer - having their foot there meant they could brake more quickly.
Wrong. It is not safer. In fact you're more likely to cause an accident by doing this, in one of three ways. First, when your foot is resting on the brake pedal, there's enough pressure that the brake light switch is engaged. Everyone behind you sees you driving along, but not slowing down, with your brake lights on. When you do finally slow down, there's no indication of that and you'll get hit from behind. It's called "cry wolf" - your brakes lights have been on for so long that the driver behind you no longer believes them.
The second method by which you're going to cause an accident is because of the lack of calibration in your left leg. Muscle memory in your right leg knows, from driving experience, how much pressure to apply to the throttle and the brake. Your left leg, by comparison, is normally either used for the clutch (which is much heavier than the brake in almost every car), or your foot would normally be sitting on the foot rest if you drive an automatic. Either way, this means if you do use your left foot to brake, you'll step on the pedal too hard and either lock the brakes, or cause the ABS to come on. This will take you by surprise and you'll do something silly resulting in a crash.
The third method by which you're going to cause an accident is because your brakes are getting hot. The weight of your foot on the brake not only turns on the brake light switch but it will also be lightly applying the brakes all the time you're driving. Not enough to slow you down, but enough that the pads are touching the rotors. This makes the pads and rotors get hot, which makes them expand. In severe cases, this can (and does) cause the brakes to lock unexpectedly. This particular condition is easy to overcome by taking your foot off the brake pedal when it happens, but typically if you're minding your own business and your car suddenly brakes hard and you lose control - it will take you by surprise and the last thing you'll do is take your foot off the pedal.
So take that left foot off the brake and put it where it's supposed to be.