Why aren't people grateful? Why are they so annoying?

The hard and cruel truth is she is not your friend. So try to ignore her. It might be hard to put her aside but you will find others that like you. People are not evil, as a rule, but this girl might be. Maybe with summer you can totally ignore her. Don't text her, don't give any "farewell" speeches, don't try any "get even" stunts, just write her off. Her failure to thank you for the chocolates shows that she is selfish and cruel so when you cross her off your "friends" list, your life will get better.