1. Sincerely -
Meaning: You really didn't have to do what you did for me, but I really appreciate it.
Example: I missed school today because I was feeling sick. My classmates, Jason and Amy, stopped by my apartment to bring some food for me.
Amy: Hope you are feeling better now. We brought some chicken soup for you.
Me: You really shouldn't have (gone out of your way to get me the food, but I really appreciate it.)
2. Angrily -
Meaning: What you did to me is something you should never have done.
Example: My sister Jane went out last night wearing my favorite white dress without asking me if she could borrow it. Then, she accidentally spilled red wine on the dress. The next morning, she showed me the dress.
Jane: I am so sorry about this. I know I am not supposed to go into your closet and borrow stuff.
Me: You really shouldn't have (gone into my closet, borrowed the dress without my permission and ruined my favorite dress.)
3. Happily -
Meaning: You did something very nice for me and I really like it.
Example: My lovely grandmother bought a brand new BMW for me as a college graduation gift.
Grandmother: Here are the keys to your new car.
Me: You really shouldn't have (bought me such an expensive gift.)