The three factors are : 

1. Terrain: The plains have encouraged higher density of population as compared to mountain region. The steep slope of the Himalayas restrict the availability of land for agriculture, development of transport, industries and other economic activities which may tend to discourage concentration of population and its proper growth. So, the great plains of North India is a land of extremely gentle slope and offers great opportunities for the growth of agriculture, transport and industries.

2. Industries: Industrial growth offers massive employment opportunities and acts as a great magnet to attract people. This results in higher population density. Industrial areas are invariably associated with areas of high population densities. One of the major causes of population density in West Bengal, Gujarat and Maharashtra is the phenomenal growth of industries.

3. Urbanization: Urbanization and population concentration go hand in hand and are closely related to each other. Agricultural based rural areas do not provide employment to all the people living there. Even small scale cottage industry fails to provide employment to the entire rural folk. Urban areas provide vast scope for employment in industries, trade, transport and services.