United States-centric. I understand they were originally formed at different times and for different reasons, but by the time the American Revolution came around, they were all fighting for independence at once, to the point where they formed one country. Why by that time were they not just "British North America" with provinces like the area that would become Canada?
Unity was not something that came easy to them! Because the colonies had been individually chartered without much knowledge of the actual geography, there had been significant disputes over boundaries, and some of those remained- New York and New Hampshire both claimed what's now Vermont, for example. And there were overlapping claims on the territories to the west, as well. There were regional differences- the planters of the South had different interests than the Boston merchants. Even within state borders, there were divisions. In North Carolina, for example, the wealthy planters in the lowland east had total control of the state government, and mostly ignored the poorer settlers in mountainous west. Lastly, although "unity" sounds pleasant, it also requires diffusion of authority- if the states of , say Maryland and Virginia combined, the governors and legislators of Virginia would in some ways have to take into account what the governor and legislators of Maryland want. Surrendering power is seldom done willingly by people, especially politicians!
The Revolutionary War was the first time, really, that all those 13 colonies had a very good reason to unify. Although they were the first to use the name The United States, the Articles of Confederation were a very short agreement that mostly limited itself to describing how the states were going to cooperate to fight that war. The Articles didn't create a real executive branch. The Continental Congress, that was the only national government ,couldn't levy a national tax, couldn't easily do treaties and diplomacy. It was only after the War that the United States realized it needed to have something more of a national government, which led to the Constitutional Convention in 1787. Even then, the creation of a powerful Federal government was controversial: there were a number of Anti-Federalist politicians who correctly saw it diminished their own authority, who wished for a much more limited national government. And the dispute over the limits of that national government continued into the 1800's. Especially over the issue of slavery: as the US had both slave-holding and free states, the Federal government had to mediate the division, deal with two different sets of laws within the same country. That was impossible to easily solve ( the Dredd Scott decision by the Supreme Court was a futile attempt to solve the problem by essentially making slavery legal everywhere) and would eventually lead to the Civil War.