American colonists' strong principles were the main reason they decided to declare their independence from britain.

The answer is, To argue for, there's the obvious, which is that the colonists didn't like their lack of voice within British Parliament. They also didn't like how Britain was treating other colonies, what would become Canada for instance, with similar laws and codes seen in the US colonies. You could also argue that it was a more financial decision, as the taxes placed upon the colonies and the restrictions on trade were hurting American businesses. Honestly, an argument combining the two would probably be more rounded.