The competition is the condition, in which someone proves the superiority over others. In nature, the competition is one of the greatest factors responsible for the evolution especially natural selection.
The resources in the nature are present in a limited amount. The resources such as water, air, land, habitat, and the food are shared by many organisms. As these resources are limited, there is a fight between different species for the same resource. In this condition, the organism, who is able to prove its superiority can survive better.
Hence, the main reason of competition is limited availability of the resources.