It is presumed that Smaug hears about the treasure within the Lonely Mountain and decides he will take it, but as the dwarves had been mining there for a long time there must've been a significant time period where the treasure would presumably have been worthy of Smaug's attention but he didn't either know about it or feel willing/able to take it.
What was the trigger for Smaug to attack at that particular time?
This is really all we know:
But Thorin I his son removed and went into the far North to the Grey Mountains, where most of Durin's folk were now gathering; for those mountains were rich and little explored. But there were dragons in the wastes beyond; and after many years they became strong again and multiplied, and they made war on the Dwarves, and plundered their works.
[..]
So the rumour of the wealth of Erebor spread abroad and reached the ears of the dragons, and at last Smaug the Golden, greatest of the dragons of his day, arose and without warning came against King Thrór and descended on the Mountain in flames.
-- Lord of The Rings, Appendix A, section III
The dragons had been at war with the dwarves in the Grey Mountains since before Erebor became fantastically rich - and the dwarves had gone to the Grey Mountains from Erebor (after mining the Arkenstone).
So from that we can conclude that either the treasures of Erebor must have been much smaller than those the dwarves found in the Grey Mountains, and the dragons would only come down after all of those had been taken, or Erebor's gold and gem veins lay deep and had been exploited only for a relatively short time, until eventually Smaug heard of them (presumably from Dwarves in the Grey Mountains who traded with Erebor).
Keep in mind that Dragons live basically forever, so a century or two means little to them.
Smaug's age is one possible answer. In Chapter 12 of the Hobbit (Inside Information), he says
The King under the Mountain is dead and where are his kin that dare seek revenge? Girion Lord of Dale is dead, and I have eaten his people like a wolf among sheep, and where are his sons' sons that dare approach me? I kill where I wish and none dare resist. I laid low the warriors of old and their like is not in the world today. Then I was but young and tender.
(Emphasis added.) My reading of this is the Smaug was relatively young when he attacked Erebor. Coming of age, he proved his prowess and gained a large amount of treasure by attacking the greatest (and richest) remaining Dwarf kingdom. Had he attacked years before, he would have been too vulnerable to arrows, or too small to incinerate his enemies in sufficient numbers.
The alternative is that 'warriors of old' whose like 'is not in the world today' refers to individuals alive much earlier, but that seems unlikely because there is no mention of Smaug in the Tale of Years before his attack on Erebor in TA2770. Moreover, had he slain some mighty ancient warriors, he would name them when boasting, rather than talking about Girion of Dale.
Smaug's taking of Erebor may have, consciously or not on Smaug's part, been due to the influence of Sauron. In the appendix to Return of the King, Gandalf was reflecting on how devastating a dragon could be on the northern parts of Middle Earth, once Sauron made his move down in Mordor. And then coincidentally, this dwarf guy named Thorin started talking about Dwarves re-taking Erebor....
