Why did North Korea launch a rocket and Japan protested?

First of all, there is no essential difference between rockets and long-range missiles. Having the ability to launch rockets means that North Korea can launch long-range missiles.

Some time ago, North Korea's nuclear test has been sanctioned by the United Nations. Under international pressure, North Korea did not continue its nuclear test, but it must have been developed in secret.

Therefore, long-range missiles add nuclear weapons, so North Korea has the strategic power to deter South Korea.

In fact, not only the Japanese protested, but also China opposed North Korea's satellite launch. If you understand the international environment when China first launched Dongfanghong-1, you will know why North Korea opposed launching satellites internationally.