People think driving is a competition, they think it's a race even though everybody's going to separate places. They think if they let a truck merge ahead of them that they are one position back, that they've lost another second on the lead car (whoever the fuck that may be). That's why people will pull out in front of you even though there's no one behind you.
Next time you're on the interstate put your car in cruise control, without fail you will gradually approach a slower car, right before you rear end them go to pass, they will speed up. Or watch a car beside you get within 2 inches of the bumper of the car in front of them just to try and stay ahead of you.