When you come up behind a 40mph driver on an open road do you smoothly and safely overtake him as soon as possible or do you become the "Second Slowcoach"?
A typical passenger car is 15-20ft long so overtaking a 40mph driver at 60mph will take around 12 seconds and 1,000ft of road. On a clear stretch of road, no junctions, driveways, sharp bends or oncoming traffic, in good conditions, that's quite manageable, quite safe.
Not everyone does that when they have the opportunity though and there isn't always a clear road either. If the road's not clear then you have no choice but when it is clear and you choose to sit behind the leader, you become the problem.
Assuming the standard following distance of two seconds between the leader and second car, a third vehicle has to choose between overtaking both vehicles in one go or forcing himself into the gap between the two. The double overtake will now take about 24 seconds and cover 2000ft of road, well over a third of a mile, a much harder proposition.
This is the beginning of what most people call "traffic" as in "I'm stuck in traffic and can't get to work on time". Traffic isn't something you are stuck in, you are traffic!
Sometimes the slowcoach at the front of the queue is just a useless driver, sometimes they're slow for good reason, they might already be going as fast as they can. The second slowcoach turns a mere inconvenience into a traffic jam; the second slowcoach is the main problem. Don't let that be you!
If you really can't bring yourself to overtake the leader, drop back, leave a bigger gap between you and the leader so that anyone wanting to overtake can make two simple passes instead of one dangerous one. It's all part of driving with due care and consideration.