America was founded by a bunch of brits wanting to escape british rule, thats probably what started it. the following years of 'pushing and shoving' between us got a bit 'out of hand' and we had a couple of minor wars (we lost, yippee). Plus all that tea they chucked in boston harbour, we've never forgiven them.
Also, britain did the whole empire thing a while back, america seems to be trying to follow in our footsteps without realising that it isnt a good idea right now.
No doubt someone will correct me on my details/semantics but this is roughly it.
p.s. its similar to the britain vs. germany/france/spain/everywhere else tension. at some point in our history we owned pretty much the world so we tend to be snotty when the colonies start acting superior (i.e. like us)