First, we should note that many people would argue that US policies did not "force" the Japanese to attack Pearl Harbor. This is an interpretation, not a fact.
If the US policies did in fact force Japan to attack, it was because they forced Japan to seek an empire. Japan did not have much in the way of resources. In particular, they had not oil or iron. They imported much of both from the US. When the US cut off their supply of these things, Japan had to either attack Pearl Harbor (so the US could not oppose them as they took places like Indonesia) or to just give up and let the US tell them what to do. This choice forced them to attack Pearl Harbor since no country will willingly allow other countries to tell it what it can do.