Answer:
It was similar to a breakup letter, because it:
1) Stated that she (the colonies) was breaking up with him (Britain) and said that she was going to say why.
2) It stated what a "perfect relationship" was supposed to be like.
3) Said what he did wrong
4) Stated what "breaking up" meant (i.e. Don't talk to me anymore. I don't want to see you.)