Why Brexit happened?

In 2016 the Brexit referendum took place and 51,89 % of participants voted to leave. It is important to learn the reasons of such result, so other countries would not repeat the same mistake again.
One of possible reasons – poor living standards in some of the UK regions. Look at the EU regions GDP (PPS) per capita of 2007:

In general in 2007 the poorest UK regions were:

• West Wales and the Valleys
• Cornwall and Isles of Scilly
• Tees Valley and Durham
• Lincolnshire
• Shropshire and Staffords hire
• South Yorkshire

And now look at the map of 2016. The UK still had some poor regions:

• West Wales and the Valleys
• Tees Valley and Durham
• Cornwall and Isles of Scilly
• South Yorkshire
• Lincolnshire

As you can see 5 UK regions were poor in 2007 and nothing much improved until 2016. As the living standard did not improve – people started not to believe in the EU. One of such regions is Lincolnshire ­– the most pro-Brexit. "Leave" was won by a thin margin (51,89 % vs 48,11 %) – these poor regions might have been the major reason. The EU leaders were focusing too much on the cohesion of Eastern Europe, while some regions in Western Europe (UK) were suffering.

FAQ

The southern parts of Spain, Portugal and Italy are poor and they do not want to leave the EU. Why?
Most of these regions live on tourism, and they would prefer to stay in the EU, because it helps to attract more tourists – they can travel more freely.

Will other countries leave the EU?
Most of other poor regions, for example, south of Belgium, east Germany experienced some improvement of living standard from 2007 to 2016 – Belgium and Germany are unlikely to leave the EU. (Update 2020-11-10: unfortunately according to the latest Eurostat map the south of Belgium experienced no improvement, however, they may still want to stay within the EU – they may want to commute to the rich nearby Luxembourg). Eastern European countries are also unlikely to leave the EU, because they get more than they pay. Portugal, Spain, Italy, Greece are also unlikely to leave the EU – the EU helps to attract more tourists.

What EU leaders can learn from this lesson?
In future the EU leaders should keep monitoring the map of EU NUTS GDP per capita. They should react as soon as new poor regions emerge in Western Europe.