Imagine how you would feel if someone older than you (maybe an older sister or brother) kept telling you what to do all of the time and kept taking more and more of your allowance. That is how the colonists felt in the years leading up to 1776.
Great Britain kept trying to make the colonists follow more rules and pay higher taxes. People started getting mad and began making plans to be able to make their own rules. They no longer wanted Great Britain to be able to tell them what to do, so they decided to tell Great Britain that they were becoming an independent country. (To be independent means to take care of yourself, making your own rules and providing for your own needs.)