A British general called the relationship a "delicate hotmess." Before the invasion the number of American GIs waiting in England had grown to over 1.5 million spread out in over 1200 camps and 133 airfields. In a War Department brochure given to each newly arriving soldier stressed that this was not time time to fight old wars and bring up old grievances.

Differences in pay between the British and Americans caused resentment as an American private earned triple to what a British private made; an American staff sergeant made equal to what a British captain made. The British did supply 240 million pounds of potatoes, 15 million condoms, 80 million cookie packets and 54 million gallons of beer to the gracious Americans.

The British put up with them even though a survey revealed that less than half of British citizens viewed the Americans favorably. A British housewife said

They irritate me beyond words. They are loud, bombastic, bragging, self-righteous, morals of the barnyard and hypocrites.

Bad behavior didn't help the stereotype. GIs in Newcastle ate the royal swans at the Kings summer palace. Paratroopers used grenades to fish in a private pond. Bored soldiers set hay bales on fire with tracer rounds. So many GIs impregnated British girls that the US government agreed to give local courts jurisdiction in "bastardy proceedings".

With all that said a British major wrote, "they were the chaps that mattered, we coudn't possibly win the war without them."

The Guns at Last Light: Rick Atkinson

John Keegan relates in one of his books (I think "Six Armies in Normandy") that the English, although happy to have the Americans in the war on their side, ruefully (and perhaps semi-jokingly) said of them that the problem with Americans was that they were "over-paid, over-sexed, and over-here."

From the perspective of young English servicemen there seems to have been a bit of envy and resentment about the competition for the attention for female company that the Americans represented. The US military seems to have been more lavishly and generously equipped, supplied, and paid than their British counterparts. Keegan also relates in the same reminiscence that the fabric used on American enlisted men's uniforms was of a quality that the British could only afford for officers uniforms, and that unlike British servicemen US soldiers always seemed to have gum or candies on them (perhaps supplied to help them get on the good side of the locals).

Given that US troops brought back a substantial number of British women with them as brides after the war, I think it's reasonable to guess that at least a significant portion of the young female demographic was happy enough to socialize with to what them must'be seemed like exotic foreigners.