1) They viewed the U.S. Pacific fleet as a serious threat to their ambitions in the Western Pacific.
2) They reasoned that if they could knock out that fleet at the very beginning, they would then be free from U.S. interference in those ambitions.
3) They foolishly underestimated U.S. resolve--they figured such a blow would be so demoralizing to the U.S. that it would give them sufficient time to establish impregnable defenses. They thought (hoped, really) that the U.S. would see such defenses and essentially give up and not contest the Japanese for control of the Pacific.
4) For the Japanese, everything depended on it being a short, lightning fast war, essentially over before it could really get started. They dismissed the importance of supply & logistics, thinking that the power of Bushido was enough, and they ridiculed the industrial potential of the U.S., which in the end would completely overwhelm them.
5) Certain Japanese officers, especially in their navy, including Adm. Yamamoto (who planned the Pearl Harbor attack) knew what they were getting into, and warned their leaders not to attack the U.S. But Army generals of the Imperial Japanese High Command ran the government, and those officers were the same ambitious, fanatics who had overseen the previous decade's aggression against China (a war which was the reason for their designs in the Pacific). They knew little about the U.S., didn't think they NEEDED to know anything about the U.S., and dismissed the U.S. as a serious opponent. Their foolish arrogance doomed their whole nation, leaders like Tojo and Anami deserve nothing but the contempt of history.