Japan wanted to seize the Pacific Islands because they wanted the natural resources of the land. Repetitive conflicts with China and Korea had depleted its many supplies of rubber, iron, and oil and, with no allies in sight, they believed seizing land for resources was the only viable option
For the same reason the US did, although the US also wanted the so hat Japan couldn't have them. Japan was in need of raw materials (particularly oil) so that they could continue their war against China. The Pacific Islands had an abundance of unused raw material, including oil. Japan probably also wanted to gain more territory.
