The Kaiser wanted Germany to enjoy what he saw as its rightful place in the world. He thought that Germany should be a major power in Europe (and thereby the world). The phrase that is typically used is to say that Wilhelm II wanted Germany to have a "place in the sun."
At the time, the most important power in Europe was Great Britain. Wilhelm II was the grandson of Queen Victoria of England and wanted to be the ruler of a power that was as strong as that of his grandmother.
Overall, then, Wilhelm II's desire to build Germany's military was driven by his ambition to have Germany be a major power in Europe.