There are several different experiences that unified the colonists in British North America during the 18th century. Here is a list of just a few:
1) Implementation of taxes by the British government- Taxes like the Stamp Act, Tea Act, and Sugar act are just a few examples of laws that united American colonists in anger against the British government. These colonists felt these taxes were unfair, as they were implemented without hearing the opinion of colonists. After this, the rallying cry became "no taxation without representation."
2) Boston Tea Party- This destruction of British tea by American colonists showed their unity against the unfair Tea Act implemented by the British government. Colonists all over the 13 colonies supported this action, as it made a point to the British government that colonists were displeased with the laws being passed in their society.