The main reason why the colonists wanted to break away from Great Britain was "taxation without representation." Great Britain were taxing the colonists, through things like the Stamp Act, and the colonists felt like this was unfair. They did not get a say in what they were being taxed on and how much it was. They had no representation in British Parliament and had no way to make these decisions, so they felt it was unfair to be taxed. This made the colonists end up boycotting the products that were being taxed, leaving Great Britain with not as much money to pay war debts. Another reason was Great Britain sometimes made large armies stay with the colonists, who were normally armed. Great Britain then made the colonists provide housing for them, along with cooking, and even paying for them.
I'd think that one advantage of nationalism is the unity it builds among people who share the same nation, who start to think of each other as co-citizens. One disadvantage of nationalism is the hate can be directed against outsiders.
