The northern areas and the southern areas of India face different types of climate (India has both tropical and sub-tropical types of climate).
The peninsular regions experience a slightly less extreme climate as compared to the northern areas due to the moderating influence of the oceans.
However, monsoon acts as a unifying force in these varying climatic conditions. There are so many festivals in India associated with the monsoon. The monsoons affect the harvest of the grains consumed by all Indians each year. Thus, each year all of India eagerly awaits the arrival of the monsoon.
and it acts as a unifying bond 
A)It provides a rhythmic cycle of season and the arrival of monsoon is eagerly waited by all.
b) Indian landscape, its animal and plant life, its agricultural calender, festivals,etc revolve around the monsoon period.
c)There monsoon winds bind the whole country by providing water to set the agricultural activities.
d) The river valleys which carry this water also unite as a single river valley unit.