"Do not speak to me of dragon fire"

[Zigûr]

It turns out (perhaps unsurprisingly) that a quick Google search revealed the answer:

From http://scifi.stackexchange.com/quest...ng-with-thorin

This is discussed in the Director's commentary. Thranduil's prior encounter with dragons (which the Director happily admits is not from the books) was included to explain his deep isolationism and the reason why he wouldn't help the dwarves. There's also the suggestion that a firedrake killed his wife, explaining her absence from the film.

As to the sudden appearance of his scars, the implication is that he normally uses Elven magic to disguise them. To emphasise his little speech, he allows this 'glamour' to drop momentarily.

Peter Jackson: What's interesting is that Thranduil's had this encounter with dragons before ... and his wife was ...

Boyens: There's a conceit we came up with. Actors need that. They need to feed on that backstory. Where did this isolationism come from?

I also like the notion that this very 'closed off' character is also one of the greatest fighters on Middle Earth.

Seems like a classic case of "we don't understand Tolkien's complex/believable characterisation so we made up an over-the-top silly one ourselves."