How and why did the separate british colonies (and later states) form a tighter and tighter unit in the period from 1754 to 1815 ?

The separate colonies formed a tighter and tighter unit because they didn't want to be apart of British rule anymore, and all shared similar ideas which brought them together over time. They did not believe in the taxation and British control that they were experiencing especially when they had no say in British parliament. The Boston Massacre was a huge turning point in uniting the colonies, as that is when all of them realized they wanted their own independence away from Britain. They also realized that strength was bigger in numbers and if all of them were together they could break away from Britain's rule more quickly and effectively.  

it took two men to wrestle rome back from chaos and turn a republic into an empire. in the first century bc, rome was a republic. power lay in the hands of the senate, elected by roman citizens. but the senators were fighting for power between themselves.