When we award the difference in value between a perfect hand and a hairy, ugly hand that causes embarrassment and diminishes job prospects, we Hawkins in as good a position as he would have been in had the doctor kept his promise.
So, if we let Hawkins keep the doctor's fee, we overcompensate him (Hawkins); we put him in a better position than he would have been in had the doctor kept his promise.
How does the expectation measure avoid such over-compensation?