Western Europe has high density of population
Answers
Western Europe is densely populated because for many centuries now it has been home to the leading nations of the world.
The British, French and Germans were at the forefront of the world's development, with the industrial revolution and the huge empires they had amassed, they basically had global hegemony and huge wealth.
With this wealth and technological advancement, the people lived in prosperity and were able to live longer and better lives than the rest of the people of the world, which meant that their population accumulated as instead of dying the elderly were still counted in the census and population figures for longer.
Also, during this time, they were in Stage 2 of the DTM, as they had very high birth rates and lowering death rates.