When the United States was founded, one of the big issues discussed was how states would relate to each other and to the federal government. As far as states relating to each other, it was noted that the northern states had higher populations but smaller areas and products. There was a compromise decided upon where there would be two houses of legislature, one that represented each state equally (the Senate) and one that represented the population equally (the House of Representatives). That concept flowed over into the states' control over the Administrative branch, too, by having the same number of electors per state as congressmen.

Remember that the world was much bigger back then. Mass transportation hadn't really been invented yet. Before the current Constitution was written, the states largely functioned as separate countries, imposing tariffs on interstate commerce, etc. The founders didn't want that to happen, but the states were still interested in being independent from each other, and that made a lot of sense then. Despite the fact that the US is physically much larger now, and has a tremendously greater population, there's much more interstate commerce and communication and the country now works much more as a unified whole than it did when it was established. State's rights may be a slightly outdated concept. After all, Europe is conglomerating in a very similar way to the way the United States are, and they're much more different there than here.