Answer:
Because the living things need to survive somehow.
Explanation:
Malthus theory of population mention that this will increase in a geometric way, meanwhile the food to feed population will increase in an arithmetic way, so in one point, people will fight for food to survive.
Later, Darwin took this concept of struggling for surviving, how a couple of living things normally have double or triple of offspring, that means that by passing the time, there are more living things that the food that they can reach. He thought also, how a living thing can survive to the different conditions of the environment like heat, food, preys, etc. Later Wallace had the same conclusion about the natural selection but using different methods.
Hope this info is useful.
