Why did European colonialism start?

Trade had been going on between East and West for a very long time .Eastern exotic goods like spices silk, precious stones etc were in much demand in the west at that time and a flourishing trade existed between the east and west through a land route that passed through Iran .

The monarchies of Spain and Portugal commissioned some of their adventurous sailors to find new trade routes to Eastern countries like India and China through sea .Such voyages led to accidental discovery of other countries/continents like Australia , Americas and Caribbean .

At first they merely setup trading posts but fearing competition from rival European powers and observing that Industrial revolution had given them overwhelming superiority in terms of Weapons(Guns) and Transport(steam ships,railways) against the natives they started capturing and colonizing countries .

As Industrial revolution had not set in yet in these countries they served as an excellent source for raw materials and a dumping ground for excess produce from their own countries .mass production was a hallmark of Industrial revolution .

Soon other countries joined in and after the defeat of Spain in a series of naval battles Britain started carving out its own empire .

By the early 18th century colonies abroad had become a matter of prestige and a show of strength for European countries against each other .

Edit: The Pope also encouraged Colonialism as a way of propagating the christian faith . Initially this was also a significant factor but as the competition set in he was soon ignored .