Why is it that some people don't like being overtaken on motorways?

I often encounter the following scenario -

The car in the middle lane (and the type of driver I'm referring to only ever drives in the middle lane) is doing an average speed of 65mph.

I am doing an average of 70mph, and therefore catch them up and move into the outside lane to overtake.

As I draw level with them, they accelerate to keep level with me. Even if I increase my speed, they will endeavour to keep up with me, increasing their speed up to 80mph or more.

After a while they will ease off and drop back to an average of 65mph so I can finally get round them.

Wankers!

Its annoying as feck. Lorry drivers often do the same espacially when another lorry attempts to overtake. The lorry on the inside does his damndest to travel in parrallel with the other lorry for ten minutes while the rest of the drivers queue up behind.

chunts