Question

Western Europe has high density of population

Answer 1

Historically the regions with the most fertile lands, good climate and a lot fresh water had high populations.

That is why in Egypt, Mesopotamia and China started the first civilisations.

That is also why France was much more populous then any other European Nation in the 18th century.

Agricultural technology helped as well. The Chinese, the Romans and the Inca had such technological developments.

In the 19th century the industrial revolution gave Western European nations a significant boost. Industrial fertilizer increased output and allowed to feed more people. Trading manufactured goods against grain and other food allowed Western countries to have a higher population than their land could support.

Low child mortality and high life expectancy gave the Western European countries a temporary boost in the 20th century. Migration might have added a bit recently.

Still climate, fertile land and rivers are the best indicator for a high population density.