When I complain it is usually about something I disagree on. There have been many of times that I disagreed in China so yes I did complain. It's like anywhere else if there is something that I disagree with I will disagree. It's not just China it could be anywhere. And yes I will miss China for the positive things they have here. Everything here is not negative! 

The human brain focuses/ is tricked on focusing on good memories (excluding extreme events) when recalling memories.  It's why, dispite the hardships they faced, your grandparents will tell stories of how great their childhood was.  Or the thought of spending the holidays with the family brings warm fuzzys to your heart, though the actually experience is just so-so.

DaBen is right, but I'd also add the factor of how long you stay. We like China, but if we stay a long time the newness wears off and then it's just the norm. Kind of like before kids go home for Spring Festival they're excited, but when they come back, they say it was just so-so. It would probably be more enjoyable for them if it was just a week or two and not a whole month.

When you've just arrive, everything is interesting, and the little nuisances are easily brushed off. but after a while they really get under your skin. And since they are plentiful, its easy to get distracted by them and forget the reasons you came in the first place and the reasons you stayed.