When the seven years' war (french and indian war) began, most american colonists were extremely proud and happy to be british citizens, part of the world's greatest empire. when it ended many of them no longer felt that way, even though the british empire was more powerful than ever. why?

Toward the finish of the war, the Colonists had discovered that England was too far away to truly represent them. It took a long time to get a message over the Atlantic and with England involved with a world war, there wasn't a great deal of outside help coming in to help the Colonists. 
The finish of the war likewise left England profoundly in the red. It raised duties and began to uphold laws that had been on the books however never implemented. Having battled the war and furthermore ventured into the red the Colonists didn't see that as reasonable. 
Last, England increased immense regions of domain from France. The Colonists needed to extend westbound however were forestalled by the English government. They would not like to have the Native Americans begin a moment war.