I have been thinking about this concept recently.
In a relationship, friendship, or job, you are expected to do A and B. If you don't do A and B, you are dropped or fired. If you do A and B and sometimes C and D (just to be nice or help out), from that point on, are the expectations for you raised? Are people let down when you do not accomplish A, B, C, and D? Even though you are only supposed to do A and B? Is doing more work better, or should you just do what is expected of you?!