Why did japan want seize other countries?

If I were to give a very general answer, this is the concept of imperialism/expansionism. Humans seek to improve themselves by nature i. e. rich people want to become richer. In the same way, Japan (and actually any powerful country) would want to expand and colonise/seize other countries to improve their global standing and become more powerful. (Imagine if Japan seized every country - we would be under their control! - just a thought haha)