There were two main reasons why Japan was motivated to expanding its power and conquer new territories, the lack of natural resources and the military superiority.
Japan was desperately lacking in natural resources as the Japanese islands are relatively poor in them. In order for Japan o be able to develop further it needed a lot of natural resources, and multiple hot-spots for natural resources were in the surrounding area, thus they set their sights on them to manage to control them.
Also, Japan has reformed its military, it had modern weapons, excellent training, and large army forces. Considering that all of the surrounding countries were relatively weak and much further back in military development, Japan saw them as easy targets, and indeed they were.
