Yeah, I don't think Americans hate the French, they just like to make fun of them.
They're rude
They smell
They're arrogant
They all smoke too much
The French & Indian war
WWII- and I'm not talking about being cheese-eating surrender monkeys. They wouldn't have needed to surrender so much and so quickly if it wasn't for that ultimate fail of a national defense strategy known as the Maginot Line. "Hey, let's build a really fortified defensive line between us and Germany so they can't attack us again!" "But what if they go around?" "Huh??"
Most Americans' only exposure to French stuff is going to Louisiana with their effed up legal system and "culture" that degenerated from some French people a couple hundred years ago
There's lots of reasons, really. Some of it is American stupidity, like not realizing that renaming "French fries" to "Freedom fries" was epically stupid b/c they're not "French fries" with a capital 'F' they're "frenched fried potatoes" with a lower-case 'f' and a completely different meaning.
Also, as fun as it is to make fun of the French, it doesn't hold a candle to making fun of the French Canadians. French Canadians combine all the LOLs of the French plus all the LOLs of the Canadians with none of the culinary contributions of the French and none of the late-90's boobage contributions of Pamela Anderson plus all of their own LOL-worthy threats of secession.