Ok, sensible answer.
London is not dissimilar from many other cities around the UK (or world for that matter). For many, not liking London would be similar to saying not liking any other large city. London being generally larger than the rest of the UK cities, it is amplified.
I have been to most large UK cities and it is definitely on a larger scale and with a higher density of people. What it does have is dozens of centre within it, some being much larger than standalone towns around the UK. You want to go for dinner in London, there's probably upwards of 50 independent areas to find a mix of places to go.
In terms of culture, there are more and larger museums and galleries than pretty much anywhere else. Add to that those that only exist in London (Buckingham Palace, Westminster etc).
With regards to transport, it's incredible. Yes, it’s busier than a town or the country but it's a city. The tube system is second to none. You can walk out of your building and be at the edge of the city in 45 minutes. This is because in central London you're rarely more than 5 mins walk form a tube and rarely waiting for more than 3 minutes for one to arrive. The cycling infra is growing massively.
In terms of employment, yes there are other hubs but finance, web and design are centred in London. Apart from a few tech companies up the A4, the big boys are predominately in London. As are the banks and insurance. Yes, London is expensive, but the average wages here are much higher. For sure you can do very well outside London, probably better than many here. But if you want to come and make your name in many industries, then the density of opportunities here is higher than anywhere else.
As for people being miserable. Probably. But only to strangers. There's 8m here. Around 4m turn up every day. Am I going to be chatty to them? No. We're friendly with those we live near - there's plenty of towns and places that even feel like villages here. We just limit who we interact with.
As for the people, something like 200 languages spoken here. What a mix. That freaks some people out. It takes a mind-set to get used to it but it's enriching. It's a welcoming place. You can really be what you like here. Nobody really cares. Inclusive.
The biggest problem is wealth distribution. London is not cheap. But that's no different to anywhere else. It's amplified because of the high density so the wealth and poverty are nose to nose.
So, looking at the above posts, many are...I'm coming to see something that London has to offer. Many are coming for work. That is because it is a centre of culture and business. You may not like it here and look forward to leaving, but there's a very good reason why you're coming and that's because the work you are coming for or the theatre or museum you are going to is here and nowhere else.