I'm not sure at what point I switched from enjoying living in London to wanting to escape as soon as possible. It was probably a slow process but I'm definitely in the latter camp now. Here are a few of the reasons...
Too expensive - For sure you can get paid more if you work in London, but this is completely wiped out by the cost of living here. Ridiculous house prices, expensive travel costs, over-priced pub and restaurant prices all add up to a fortune.
People - I don't have a problem with the people as such, although of course people up North are friendlier, but there are just too many of them. There is nowhere to go to get away from everyone, although deep within the bowels of Wimbledon Common can be pretty quiet on a weekday.
The other problem is that a lot of the people here are only here temporarily, generally to earn a bit of cash before heading off somewhere else. It's hard to have a sense of community when your neighbours are constantly changing.
Driving - I remember when I was younger I used to enjoy driving but the reason I enjoyed It was because I was able to drive without thinking too much. Driving in London is a different matter. First you have to constantly look out for idiots who are going to pull out in front of you. Then you get harassed by some idiot who is determined to get to wherever they are going in as short a time as possible. Then when you get to where you're going to you have find somewhere to park, generally an almost impossible task.
Dysfunctional weather - I have happy memories as a kid of building igloos, sledging and putting chains on the Land Rover when the snow came down. In London I have memories of having a super quick snowball fight and trying to knock together a snowman before all the snow melted by lunchtime. The ground temperature is so high, snow has no chance of surviving. Other weather works kind of as expected I guess.
Commuting - Fortunately I don't have to suffer the pain of this anymore, boy am I glad about that.