Here are my thoughts on what really drove them to the streets. Contrary to popular belief, I don't think economics plays a crucial role here. I think at least 70% of their gripes are political (since those guys are students who still get paid pocket money by their parents).
1. HKers used to feel superior. But in just 10 years the table has turned. The mainlanders are now buying our houses, buying our formula, driving the prices up while bringing nothing but their supposedly uncivilized "tǔ háo" behaviour to Hong Kong. Shop owners in tourist areas and luxury brands benefit most from mainlanders, but they are the minority. The majority feel they have nothing to gain, but everything to lose. The only thing they can hold on to their superiority is "we are freer and you know you are envious of our freedom" and "our air is cleaner".
2. Through successful brainwashing, anti-China sentiment grew. In the last 10 years, I've witnessed friends' changing opinions on China. One friend who once called China his "home country" and celebrated the 2008 Olympics, is now mocking China as "強國" (strong country) and despises China. He once commented: "China is done. I saw no hope for China anymore", which is exactly the opposite opinion the world has on China. Why? Because the best-selling newspaper, Apple Daily, succeeded in propagating the dark side of China while hiding all the positive changes. Cable TV, MingPao, HKEJ and RTHK also flocked to discredit and demonize China whenever chances they have. These grievances are in turn exaggerated and broadcasted 24/7 by anti-China propaganda and the teacher association.
Oh and this brings us to the teachers' association, dominated by a number of Pan-Dem lawmakers who hold crucial positions. They are the ones who staged protests against any form of "national education" a year ago.
3. Anti-China, or should I say Anti-CPC sentiment explains why some of the HK people are okay being ruled by a British dictatorship while not okay under a democratically elected HKSAR government under China. Their fear of the CPC is beyond rational. Paranoid is the best term to describe their emotion. Here, I'm mostly talking about the 80s and 90s generations.
4. Driven by paranoid and their deep mistrust of China, combined with the lies from politicians, they feel they can severe ties with China as much as possible and still do fine because "they did okay without China before 97 anyway".
5. For the students, economy is the least of their concerns. The 80s do have their whole set of grievances such as the rising house prices and their wages not keeping up with inflation. But they fail to realize this is the plight the whole is facing and the immediate cause that led to the Arab Spring. As always, it's as easy for the devious politicians to blame this on the government as it is for the simpletons to digest.