Young patriots with a lot of get up and go, the same melding of personal, economic and national goals that had overthrown the old order in the Meiji restoration - the same kind of people who colonized India for the British Empire. Progress and national imperatives, at any cost. These kinds of people don't stop to think of the consequences of starting wars they can't win.
Early military success leads to an unrealistic appraisal of your own capability, much like Hitler when he decided that invading Russia would be a good idea.