I live and work in Shenzhen and am doing an MBA in Hong Kong. As one mainlander told me, Hong Kong can protest for democracy as much as they want, but independence from the mainland isn't ok, that's the line that can't be crossed. Of course, the government's long-term stance on democracy in Hong Kong is more complicated and he didn't speak to me on behalf of the government.
Hong Kong is an extremely touchy subject for mainlanders. As told to me by one of my professors, the taking of Hong Kong by the British Empire was taught in mainland Chinese schools as one of the most shameful aspects of Chinese history. Hence, especially now that China has Hong Kong back from the UK, whenever there is any kind of statement that Hong Kong should go back to the UK, or that Trump needs to come help Hong Kong, or that Hong Kong should be independent, it's met with a visceral reaction from mainlanders. That was apparently a time in history when foreigners would have shops or restaurants in Hong Kong and the mainland and have signs that said "No dogs or Chinese." It's extremely easy for mainland people to get extremely emotional about this topic. The fact is that despite Mao's shortfalls, he was the first one in a hundred years to set up a government that didn't get bullied by foreign powers. That period is literally called the hundred years of humiliation in Chinese history. Due to this historical context, now that Hong Kong is part of China again, ANY foreign influence or interference is strictly viewed as unacceptable by mainlanders.
So there's a bit of a powderkeg of non-overlapping priorities between the mainland and Hong Kong lower and middle classes that make for difficult conflict resolution.
I mention specifically lower and middle classes because it does seem to be mostly the lower and middle classes that are unhappy. I guess when you're a billionaire, you can do whatever you want and don't really care what kind of system you live in. But there is very good evidence that Hong Kong's economy and people have been left behind, just as America's "Trump base states" have been left behind. Through many decades, Hong Kong industries have never really evolved. For these last few decades have been centred around real estate and finance, and that's with not much land on which to construct new residences. So prices go up while there are no jobs to pay rent. There are some crazy photo essays on the Internet of Hong Kong people who live in what look smaller than jail cells because rent is so expensive. Meanwhile, Shenzhen totally caught onto the high-tech craze, jobs are everywhere, and everyone's making money. So there's a lot of mainlander consensus that Hong Kong dropped the ball and now Hong Kong is angry without taking responsibility. But there's no Trump to vote for because the mainland government reserves the right to restrict electoral candidates, so now they're protesting instead. And mainlanders look at them and say that no democracy can solve their structural problems, or else it would have been done already. Read an article shared by a Chinese friend that talks about how the Hong Kong government set aside a ton of funding to revitalize Hong Kong with new digital industries, and instead Hong Kong's super rich people funneled that money into their personal real estate projects. They'd say any democratic solution in Hong Kong won't have real teeth because look what happened before.
There are so many complicated factors involved that the two sides aren't really listening to each other. It reminds me of Colin Kaepernick. He's protesting about police brutality against black people, but people are angry at him because they say he's disrespecting the American flag and military. When two sides are not even on the same page, difficult for them to have a conversation.
Unfortunately for Hong Kong lower and middle class people, due to their economic weakness, they don't actually have a lot of leverage in these issues. 2014 Umbrella Movement showed as much. That's why the protestors are more violent than 2014, and that is slowly making them lose the moral high ground. The biggest point in a lot of mainlander eyes was when they beat up a mainland reporter at the Hong Kong airport, thinking him to be a mainland undercover cop. Mainland social media exploded that night, a lot of mainlanders got really mad and the next day was filled with the "What a shame for Hong Kong" meme. It's a bit like Ireland in that the IRA found it really difficult to give up their guns because their guns were their only big leverage in the negotiation with the government. Similarly, Hong Kong doesn't have much else to bargain with right now, so here we are.
Anyone who thinks this issue is black and white has no chance of offering a solution that will actually work. Scared of what could end up happening.