Question

why south and north Korea are enemies?

Answer 1

North and South Korea didn't agree in the same governing plan. South Korea tried to cooperate with North America but North America refused and they split. Now, North Korea plans to attack South Korea and make it into one government for themselves.

Answer 2

When a country gets divided in two or more parts, they start to fight each other to became political dominant and to capture natural resources.

Most of these conflicts are political motivated. Political parties in these countries always try to gain masses support by relating these boundary conflict to national pride.
Same thing is happening between India and Pakistan. Even newest nation South Sudan is also in territorial dispute with Sudan.