There are various reasons for population growth. The obvious reason is that there are more births than deaths. This is called natural increase. The global total fertility rate or TFR (the number of children a woman is expected to have in her reproductive years) is 2.5. This is much lower than in the past, but still greater than the replacement level (TFR of 2.1) which would lead to a stable population.
Another major reason is a result of people living longer and lower infant mortality. This is due to advances in medicine and healthier lifestyles. These are causes for celebration. We can be thankful that most people can live to see their grandchildren and that the vast majority of children survive through the formative years of their lives.
Migration results in population changes at regional levels. Some nations, such as New Zealand, receive more migrants than they send, while others send more migrants than they receive.
In New Zealand, natural increase accounts for about two thirds of population growth and migration for about one third.