The first thing to point out, would be that there was in fact drug addiction in Europe in the 1700-1800s. The particular reason as to the wide spread use of opiates in China was that the Portuguese, British and Dutch used its addictive properties to create the demand.

The background to this information is there was a very high demand for Chinese goods in the west. In particular silks, porcelain, and tea were much sought after. The issue with trading the Chinese, was finding a sustainable commodity that the Chinese wanted. Initially it was all done in gold and silver, but without a surplus of this (such as in the United States where gold was not mined until the late 18th century). When the discovery by the Portuguese that Indian grown opium could be sold in China where it was used by almost every level of society. Soon the other parts of Europe and North America began using opium as their main trading commodity.

The point is not that silver discontinued its journey into China during the second half of the eighteenth century—it did not—but rather that opium and tea became the high-profit markets, with silver playing a complementary role in terms of profitability. According to Dermigny (1964, Vol. II, 688-689)

(Flynn & Giraldez, 2002, p. 411) Cycles of Silver: Global Economic Unity through the Mid-Eighteenth Century

Next we need to look at what opium was used for, and how it was viewed in both China and West Europe. The use of opium was both recreational and medicinal. There have been examples of opium's medicinal use throughout history, specifically as a pain killer which made its use widely spread during times of war. It was well known even before its widespread trade on the global market. Xu Boling writing in 1483 described it as an aphrodisiac, which begins to show its uses that weren't strictly medicinal. It was also used for purely recreational purposes, but would not have been easily accessible to most Chinese prior to the establishment of European opium trade in the region.

It common use as a medicine and recreational drug lead to widespread addiction, as the marketplace was flooded with the substance on levels significantly higher than what the West experienced.

"16,550 chests in the season 1831-2 to over 30,000 in 1835-6 and 40,000 in 1838-9"

(Greenberg, 1951, p. 113) British Trade and the Opening of China 1800-1842

TL;DR Western Europe and the Americas flooded the Chinese marketplace with Opium, making it extremely accessible to people in the region.

tl;dr Western Europe and the Americas flooded the Chinese marketplace with Opium, making it extremely accessible to people in the region.

Wasn't laudanum, an opiate based drug sold as a medicine wildly popular?

Secondary question: has this historical forcing of opium on China and the widespread abuse left any residual resistance to the use of modern painkillers? Does this historical artifact still manifest in Chinese popular culture?

Edit: When I say "resistance," I do not mean an actual physiological resistance, but rather a cultural resistance. Do people hesitate to use painkillers due to stigma or otherwise?

How did the Chinese became addicted in such a widespread way? Better yet, why was legalized opium so harmful to the Chinese society to begin with? Contemporary wisdom has it that the "best" way to deal with addictive substances is to legalize and regulate them. Didn't the British do the Chinese a favor by forcing them to regulate the opium trade?

As supplementary study material, Thomas de Quincey's "Confessions of an English Opium-Eater" is a quite good firsthand account from 1821 by an Englishman who was an avid opium addict.