Why do some organisms compete for food in an environment?

The simple answer would be resources. There is always a limited amount of resources, and when they are more scarce, competition is more fierce. Few animals (lions and hyenas are one example) compete out of a perceived hatred for each other (people may be another), so competition is usually about survival. Injury often equals death in the wild, so rarely will animals compete physically for food/water/territory/mating rights unless their livlihood depends on it.