Why did Britain want to have North America?

Britain wanted North America for the same reason that other European powers were busy colonizing as much of the world as they could: resources. Prior to England's move to colonize North America, the abundance of resources had already been reported by explorers. France had established trading outposts, and Spain had acquired its own territories in North America as well.
North America had abundant amounts of arable land that, as far as European powers were concerned, was unowned. The forests of Maine and Massachusetts were also very attractive because of Britian's need to maintain a navy. The tall old growth trees of the New England area were perfect for ship masts, making it a desirable area to occupy.
The final reason for the desirability of North America was the sparse population of Native Americans. While Native Americans had been plentiful prior to European attempts at colonizing, the diseases which were carried over by explorers had led to precipitous (up to 90%) depopulation due to death among Native American tribes. The abundance of resources and the relative lack of native population able to prevent the exploitation of those resources made North America a very desirable area to colonize.