What are the factors that cause alot of Americans to hate the police?

They say they are racist when the police don't come into their neighborhoods but when they do they are "racially profiling".

because all the cop haters r criminals and love to violate the law without police

most police in the u.s. are racist

and gun control is a big problem in the u.s. even kids who go insane and start shooting ppl can buy bullets at a walmart

There's been so much police brutality, racial profiling, and officers using their authority for the wrong reasons that some of us feel a sincere hatred toward police officers. You stated yourself that there is very little crime in Japan, maybe that's why or it could be the fact that everyone in Japan is pretty much Japanese or look the same. Here in AMerica we have a variety of people and varieties of beliefs. For example, a white police officer who was taught his whole life that black people were bad and to hate them, is more then likely going to overdo things when in confrontation with a black person.

because americans act out more, and so they feel like they 'need' guns. and that cycle perpetuates. also, i hear that while less crime happens there, there are some really brutal crimes that do happen. and there is more racism there, too. no place is perfect.

I think you explained the difference quite clearly.

"The police are very nice to people and are reasonable."

This is not how it works in the USA. Sure, there may be a nice cop here and there. But for the most part, cops are as crooked as a barrel of snakes. They steal drugs from people and either use them or resell them (or both). They abuse their power. They have no respect for traffic law when it comes to them having to follow the rules. They are never there when you need them and always there when you don't. If you call them for help, they come to you and treat you like you are the criminal. They look for reasons to arrest you.

Supposedly in Japan Police have a way of making themselves look good and a lot of murders actually go unreported. People in America are probably a little more free-willed culturally and don't like being pushed around.