Why the PM of Japan chaging very offten ?

Answer 1

Japanese are sensitive to issues blamed on them and when they cannot take it, they resign and this is the reason why PMs in Japan change very often.

Answer 2

PM in Japan is not a ruler like the President in Presidential system. He can't get only sympathizer for Cabinet members. He is a delegate of the diet members and people.
PM knows, once he lost the people's trust, he would lose other diet member's cooperation and the Diet will be in chaos. To avoid those disruption, PM resigns himself.
It is like the way that Samurai takes his responsibility and commit harakiri (seppuku).