The cultures are different, as much as both "Mainlanders" and "Hong Kongers" (not Hongers) are Chinese, but beyond that, the mindset is completely different. The common "western properties" that Hong Kong got as a result of the British occupation is obviously not inherent in China. Many things that the western world views as unacceptable is deemed common, part of the culture, and acceptable in China.
Examples: Spitting, squatting with your legs wide open, speaking loudly, budging, being even more ignorant and self-centered than people in Hong Kong, and quite a few others I can't seem to pin point right now.

In addition, the years of seclusion China was in led many to grow afraid of mainland China - not of its people, but fear of the ways they do things there. Stories get passed down where one small crime leads to execution, complete government domination, and general shady shenanigans. To add more injury to insult so to say, the June 4th Tiananmen Square events didn't exactly put a positive light on the Chinese government as well. The sheer brutality used against free speech, which is a basic right in Hong Kong, scared many shitless on that very day. Needless to say, the lack of development in China led the mainlanders to be viewed as "香下佬" (wrong characters?), which translates to something equivalent to "rednecks" or "villagers of the past", used in a manner similar to how the Whites called the Chinese "Chinamen" during the days of the CPR.

As a result of the brutality people perceived in China at the time, many in Hong Kong were scared of the magical date - July 1st, 1997 - the day when the British leaves Hong Kong and hands it back to China. Many families left HK as a result, including my own, immigrating to places with a western way of life such as Canada, the States, New Zealand, and so forth. Needless to say, as much as this fear was totally unnecessary, many went back to Hong Kong as '97 came and went without political problems.

Then the problems came as the HK SAR (special administrative region) government decided to push out the "自由行" (free wandering) policy, whereby permits/visas were no longer required for mainlanders to visit Hong Kong. Suddenly, floods of mainlanders came to Hong Kong to shop/hang around, and ABUSE THE HEALTHCARE SYSTEM. (ie: get pregnant, during the 9th month they go to HK to freeride the health system and to gain automatic citizenship) Keep in mind that most who do this are dirt poor, thus becoming a further social burden. Soon, the term "自由行" become a derogatory term for referring to mainlanders roaming the streets without any purpose, only to create a social burden.

This influx of mainlanders wasn't exactly helpful in improving their image as well, as HK forums started to overflow with random candid images of squatting people, horridly dressed (ie: the classic chinese lady wearing sandals and stockings), and news stories of people pissing in water fountains, shitting in washroom sinks at Ocean Park (NO JOKE) etc...

But this polarizing effect isn't solely the fault of the Mainland Chinese as well, the British Occupation and the success of Hong Kong as a financial powerhouse had a great effect on the mindset of Hong Kong people. We feel superior to the Mainland Chinese, just because our education is English based, and we are the center of Asia's financial system. The media coverage of weird events also had an interesting effect as well, with bias worsening at an exponential rate. With the amount of media outlets in Hong Kong, most pointed problems to the Mainlanders with pure speculation, adding to the alienation and polarization issue.

Oh yeah, differences in language also plays a huge role. HUMANS ARE NATURALLY DETERRED TO DIFFERENCES

Needless to say, they are changing, conforming more and more to western social standards. It's not their fault to begin with, as China did start off as a pretty poor purely communist country with conditions similar to those of North Korea right now.

AGAIN, I don't mean to offend, but it's just the way I look at the issue.