It Has Not Been Forgotten

Thranduil has been lambasted for his national and racial prejudices, which have often been exaggerated ad nauseam, making his hatred of Dwarves and Noldor almost psychopathic. Certainly he may bristle a bit, but he is not unreasonable, or else Thorin and friends would never have seen the light of day again, unable to survive long enough to escape. There were probably many other torments the Elvenking had ready to hand besides mere solitary confinement. 

Even so, his grievances were not without cause. It may be pure speculation, but based upon canon references it would seem reasonable to assume Thranduil was old enough to have been born in Doriath and witness its fall. His king (and perhaps kinsman) was murdered by Dwarves, and his home (probably Menegroth) was sacked and plundered by Dwarves:

"But the Dwarves held their way, and passed over the great bridge, and entered into Menegroth; and there befell a thing most grievous among the sorrowful deeds of the Elder Days. For there was battle in the Thousand Caves, and many Elves and Dwarves were slain; and it has not been forgotten. But the Dwarves were victorious, and the halls of Thingol were ransacked and plundered. . . . and the Silmaril was taken." (Of the Ruin of Doriath, The Silmarillion)

That would amply account for his resentment toward dwarves, an instance of what would today be called "terrorist violence." It could not have been a pleasant memory. But the Dwarves were not the half of it:

"But Dior returned no answer to the sons of Fëanor; and Celegorm stirred up his brothers to prepare an assault upon Doriath. . . . and so befell the second slaying of Elf by Elf. . . . Thus Doriath was destroyed, and never rose again." (Of the Ruin of Doriath, The Silmarillion)

With Elwing Dior’s daughter, the survivors fled to the coast of Sirion with the Silmaril, "And so there came to pass the last and cruellest slayings of Elf by Elf . . . . For the sons of Fëanor that yet lived came down suddenly upon the exiles of Gondolin and the remnant of Doriath, and destroyed them. . . . Then such few of that people as did not perish in the assault joined themselves to Gil-galad, and went with him to Balar. . ." (Of the Voyage of Eärendil, The Silmarillion)

Now, I think it is plain that the Sindar, or rather what was left of them, have been hounded quite a bit in the course of the First Age, and thus can be excused for acquiring a defensive frame of mind as a consequence. Thranduil would remember being violently ousted from no less than two homes, freely leaving two more to finally settle in Greenwood with his father, only to move again at the advent of Celeborn and Galadriel, and AGAIN in flight from the Shadow. NO ONE would move him then. He had played the pushover long enough.