I never understood why it's a dream, but you're right that for many people that I've met living in London is like the goal of their life. I moved here for the money, but the quality of life is terrible.
Apart from the obvious (weather etc), it's even hard to say. The city just feels harsh to me. One of the things that I first noticed when I moved here was how it's uncommon to find places to sit when you're out and about. If you're out all day you can't just sit in some bench and rest a bit because they're virtually non-existent. You have to go into a coffee shop. I think it's loads of tiny things like that.
Maybe that wasn't very convincing...
EDIT: Of course also the rents. I'm lucky enough that I have big windows to the park, but I'm paying an absurd fortune. The alternative is moving to the periphery where I have to spend 1 hour on the tube each way and when I go home I'm worring that I'm gonna get stabbed.
Never lived in London, but I hated Manchester. So it's not necessarily "a way better" choice, but there's a choice, which is the point I guess. (I remember Newcastle fondly.)