There are a number of areas of high population density in the world. Why does this happen?

Explanation

The areas with high population density across the world have at least one or usually multiple factors favourable for the settlement of population. Wherever people found conducive conditions for living, they have been settling there. With passage of time and growth of population, these areas became regions of thick population density. For example, the regions with availability of water, good climate, presence of minerals and other resources, of religious or cultural significance became regions of thick population. Example: Ganga-Yamuna Doab, Mediterranean regions.