I think there are a number of factors.

One would be disapproval of American policies, which can be fairly heavy-handed, as well as ubiquitous considering the global reach of the country.

Another would probably be lack of contact with the average American and general American culture, meaning that for many their main ideas about America would be based on Hollywood and American tourists, who can sometimes be very annoying and insensitive.

I think another, and probably the most important, is envy. America has one of the highest standards of living in the world, and even the poorest Americans live in conditions many people would consider luxurious. We have an incredibly high rate of consumption, and can throw our weight around worldwide both economically and politically, making us a natural target for those who wish they could live in similar conditions or take pride in their own nation's power.

There are probably more as well, that either aren't coming to mind at the moment or that it would take an outside perspective to recognize.

Ajax