I hope this was the type.of answer u was looking for.
Most European governments thought that colonial rule would end much later. In colonies like Angola, Mozambique, Algeria, and Kenya African people were forced to fight wars to win their independence.They were increasingly driven to fight for an end to colonial rule. Self-rule became the slogan.
There are more than three reasons why this happened of course but the best three are there they needed more land, resources, and some way to escape the cold.