Why did Japan become imperialistic?
It was because of culture and necessity fro that culture.
In the 1930's Japan was dominated by a militaristic government and culture, based on the Bushido code.Aggression and expansionism was part of that culture. This also reflected racism against virtually anyone, whether it was the decadent West or the inferior neighbouring Asian countries.
To be able to pursue such an aggressive foreign policy based on military expansion, the Japanese needed access to raw materials which mainland Japan didn't have. This included oil and rubber.
Their imperialism therefore was a result of these factors. expanding into China through the seizure of Manchuria, the occupation of Korea and the swift defeat of European and American colonial forces throughout South East Asia bringing Japan to the bring of Australia.
This in turn gave her the raw materials she needed such as oil from the Dutch East Indies and access to strategic bases such as Singapore. Her militarism and racism were also reflected in the barbaric nature of Japanese occupations.