Why did the chinese and japanese isolate themselves from the rest of the world?

Both nations wanted to maintain their culture as pure as possible with little to no outside contact with the outside world. This originally was done to keep out Western-world influence. They had a massive amount of internal wars during the 1600s especially! Like Samurai? They were at the the very top of the caste system right under the Shogun, which was the "real" emperor.

TL;DR: Both countries wanted to make sure they weren't "corrupted" by influence from the outside world to presenve their culture and make sure it never disappeared.