Something I've noticed time and time again...
When you put your expectations of how someone else will behave onto them and they don't fulfill your expectations, you tend to get really upset over this.
I think this is the ROOT of many many many relationship issues.
A good sign that you're doing this is when you find yourself saying (or yelling) "he should have _____" or "I can't believe she didn't _____". Fill in the blank with whatever you want ... 'send me a thank you note', 'apologize', 'pick up her clothes from the ground', 'bring his wallet', etc etc and on and on.
I see so many people putting themselves through so much anguish because of what they perceived that some OTHER PERSON "should have" done. (It's usually underscored by an attitude of "well, I would have done xyz" or "my mother always said to behave in this way in that situation" or something similar).
I notice this the most in other people. Of course, up until now, it's been more difficult to notice it in myself. I'm throwing my gauntlet down for myself right now. Recognizing it in others is the first step.