I believe its mostly self ego, stress, arrogance, sheer bloody-mindedness.......and to impress others.
So you are on a dual cariage way, normal traffic, you are going just under 70mph...you catch up to someone driving at 55mph, minding his own business, you want to overtake, but someone is coming up fast on the outside lane, you have to slow down.......who is the safest driver 

The driver driving within the limit at 55mph,
You who was driving just under 70mph and had to slow down, probably mouthing off at the same time about slow drivers
The driver coming up fast at 70mph on the outside lane, who flashes by.
Right lets look at it....
The driver at 55mph...not breaking any law, but as its a 70mph limit, should he be driving faster???
You....driving just under 70mph. Should you not realise you were approaching slow traffic and pulled out earlier???
The driver travelling fast (70mph) Should he have known he was approaching a car that in itself, was approaching a slower moving car, and would need to pull out, and eased off and flashed the other car out
(I know 'flashing headlights) is not in the highway code, but its accepted...even the police do it)
I bet all those drivers think they are the best driver one in the right....what do you think. 
You are most probably right, in this example you chose. But why can't people be humble occasionally and say "hey, sorry man, my fault". Or acknowledge the other driver with a quick apology.
So, the point still remains. If everybody is thinking that everybody else are the crap drivers, and everybody knows that everybody thinks everybody else are crap drivers; who are the crap drivers? And by what standard anyway?
You think we,re all crap drivers, double zero?

Me? I'm a crap driver. Too fast. Too impatient. ...and too intolerant.
Lady over the road, defrosts the car for half hour with engine on and all the doors open, wipers on, kids in the car belted in screaming and shouting at 7.30 to 8.30 am every morning. This morning, in addition, after half an hour of defrosting, the inside froze up as well. Doesn't do anything about it, shrugs, as if not understanding, gets in, reverses across the close, straight over the garden opposite and busts the rear bumper on the rockery. All with about 3,000rpm of throttle and almost zero clutch bite at 0,5 mph.
Bats not an eye at the collision and drives off on full lock on ice with even more revs on at 0.5 mph until, I can only assume, her foot slips off the clutch altogether and she wheel spins off up the road, arms flailing at the wheel in blind panic. All with three kids in the back.
Now I may be a crap driver, but .... Well...... You tell me?
Terbert's example, regardless of speed. Driver should not cross the white line until safe to do so. This is true if crawling in traffic at 10mph, at the limit, or well over. IMO of course. (If the driver in the over taking lane gives you right of way, that's his right, and his alone, as you are being over taken

)