The colonies were started at different times, so the motives for their establishment differed. Northern colonies like Massachusetts, Pennsylvania, Maryland and Rhode Island formed around religious and personal liberty. Massachusetts for Puritans, Pennsylvania for Quakers, Maryland for Catholics, and Rhode Island for those who found Massachusetts too restrictive. Southern colonies like Virginia and the Carolinas were more entrepreneurial ventures centered around plantation farming of valuable cash crops like tobacco and indigo. Some of these ventures, like cotton were produced also but wouldn't take off until the Cotton Gin made it profitable, and other ventures like silk (and gold mining) never took off. Georgia was started as an alternative to sending people to Debtors Prisons.
The colonies experimented with different forms of governance before they found a system of representative government that worked. The reason for the experimentation wasn't necessarily a noble pursuit of democracy, but rather the trial-and-error of discovering that many of the models that worked in Europe couldn't apply to a land with such vast open space. Maryland attempted establishing feudalism in the New World. Some attempts were made at establishing colonies run by a company, with it's people living as indebted employees. Massachusetts attempted a more theocratic form of government. All of these failed miserably for the same reason: if people felt too oppressed, they simply had the option of escaping into the wilderness.
At some point the Crown noticed this and granted them a degree of self-rule. It had been observed that when left to their own devices, the colonists would clear the land and farm it, either co-existing and trading with the natives, or driving the natives away. Realizing this, they just needed to support the colonists with troops when needed, and the colonies would more or less run themselves. This cost the British less to maintain, and the mercantile relationship proved to be mildly profitable, with raw materials being generated by the colonies that were sent to Britain for manufacturing into finished goods, which could then even be sold back to the colonists themselves. Outside of Virginian tobacco, these colonies didn't produce anything too valuable, when compared to colonies like Jamaica that had the climate to grow valuable sugar cane.
This did however allow the colonies to be self-sufficient enough to grow on their own, without constant external inputs from the mother country as had been the case before. The element of representative government allowed the colonists to handle day-to-day administration of their colonies, unlike the more profit-driven colonies in the Caribbean that were run more like companies through colonial governors or by corporations like the British East India Company. The American colonists began to feel more like British citizens, and had expectations of being treated as such, including having a say in the governing process and taxation. To the extent that their assemblies could be overruled by Parliament back in London, they began to feel unequal, which would become an increasing challenge as the colonies became more populous, more prosperous, and more ambitious in wanting to grow their own territories and economies.
Unlike under the Spanish, who limited their colonies to only trading with the mother country, the American colonies were able to trade with each other, although mercantilist policies by Britain did prevent them from trading with outside nations. However, these more local economic ties with their fellow colonies allowed the Americans to establish relationships with each other, and their common representative institutions gave them a shared framework to cooperate when needed. In some instances, disputes over issues with the British (especially when it came to taxation) were resolved by the colonies joining together to form a united front in negotiating more favorable terms, or to repeal unpopular laws.
It was one of these such disputes that would eventually escalate into the American Revolution. To find out more, and the other answers to your questions, please feel free to reach out to me to retain my services.
-Dan
