Well, this is a short one-shot that wrote itself, and I really like how it turned out. A possible reason why Thranduil didn't aid the Dwarves on the day Smaug came to Erebor.

Pride

Thranduil didn't lie to himself. True, he bent the facts somewhat so the overall image was more favorable, but outright lies? Never. But right now there was an irate Dwarf, his prisoner, glaring up to him.

The same Dwarf whom he had neglected to help when he most needed it. He liked to think it was because his warriors had been too few in number to kill a dragon, although there were more than enough to kill a large Orc-pack, the foe they'd thought they dealt with. But while preserving the lives of his people had certainly weighed heavy while making his decision, it wasn't the real reason.

Nor was the fact that the Dwarves were already defeated and had no real opposition left the deciding factor.

No, it was his pride, simple as that. Thrór, back then King under the Mountain, had insulted him by demanding he bowed before the Arkenstone, boasting his rule was divine just because he had found another pretty jewel. And though the Arkenstone was indeed beautiful, it was not enough to make Thranduil bow. In fact, it would take one of the Valar themselves to make Thranduil bow.

So, when the Dragon came he left the Dwarves on their own, looking straight in the eyes of the young Prince as he gave the sign to retreat. Knowing that someday he would come back to reclaim his lost kingdom, and knowing that if he succeeded, the relations between Erebor and Eryn Galen would suffer. And that time had come, for in the eyes of the same Dwarf, albeit somewhat older, still burned the hatred he had seen on the day Smaug took Erebor.

And as he looked in those eyes, he couldn't bring himself yield under the harsh glare, to admit to a Dwarf he had acted erroneously those few decades ago. So he ordered Thorin to be led towards the dungeons, his pride demanding he showed who was in charge.

- - The Hobbit - -

A few weeks later, when word came that the Dwarves had all escaped, Thranduil cursed his pride to the darkest pits of the Void.