Typically, organism A acts as though it wants to populate the world with its offspring, and organism B acts as though it wants to populate the world with its offspring, and these goals often conflict - because A and B have non-identical genomes.

It's very hard for A and B to know how much their genomes differ, because they can only observe each other's phenotypes, and they can't invest too much time in that either. So they will mostly compete even if their genomes happen to be identical.