Specifically, the First Opium War was caused by opium. More broadly, it was caused by a conflict between the Chinese trading philosophy and the British trading philosophy. Even more broadly, it was caused by British attitude towards the sovereignty of non-European nations.

In the early 1800s, China was essentially self-sufficient. They did not need any raw or finished material from any other country. They could afford to not buy items from foreign countries. On the other hand, foreign countries were highly interested in buying goods from China. In an ideal international trade relationship, Country A buys potatoes from Country B and sells them oranges, while Country B sells potatoes to Country A and buys oranges from them. In this situation, however, all England had that China wanted was cash. So, English merchants couldn't really make a profit on trades with China. This is called, for the English, a trade deficit.

Because China didn't need anything from outside nations, they had no incentive to make their international trading policy simple for foreigners. The Canton System was a complicated process by which foreigners could buy Chinese goods. By controlling the market, the Chinese authorities could set prices that were advantageous to the Chinese and disadvantageous to the British.

There is a concept in economics about elasticity. Generally, as price of a good goes up, people are willing and able to buy fewer of said good. For an inelastic good, demand stays the same, no matter the price. An example of an inelastic good is water, electricity, medicine, etc. Another example is a drug to which you are addicted. You are willing to buy it at the same rate no matter the price.

To balance out the trade deficit, the British began smuggling opium from their colonies in India into Chinese cities. Now there would be something the Chinese would be interested in buying from the British, making British trading missions to China more profitable, and thereby lowering the price of Chinese goods in the British market.

Of course, the sale and consumption of opium was illegal in both England and China, but the British didn't really care.

As we know, opium is highly addictive, and I've seen stats that indicated up to 10% of the entire Chinese population was addicted by the 1830s (with a higher concentration of addicts in coastal cities). Not only was this a public health crisis, it caused an economic crises due to lost productivity.

Basically, in 1839, Commissioner Lin confiscated about 1200 tonnes of opium and destroyed it. China would not compensate the British for the destroyed opium and so the British sent in gunboats.

War started.

I honestly find the cause of the first opium war to be far more understandable than the cause of the second opium war.