|Even helping in the yard can be joyful and good when you|
are at an age when you can appreciate the good in everything easily.
He quotes R' Dovid Hofstadter that good means spiritually successful; that Sarah may have had more tests from HaKadosh Baruch H- and passed them all due to her inherent goodness. Also, living through a negative experience but triumphing over it may better the one who experiences it. It leads ultimately to the perfection of the soul.
The good Rashi discusses here then is not just that which is given to us, for we know that everything G-d does is for the best; but also our own responses and behavioral learning have the opportunity to improve over and over, becoming more and more spiritually perfect. No matter what trials befall us--and we certainly can agree that even if Sarah's experiences had to be good because they came from G-d, they none the less were stressful, troublesome, with a strong potential to be appreciated as negative--they may be negative, they may be difficult, they may be painful. Our responses can fairly be negative, angry, sad. Yet simultaneously we always have the opportunity to turn each moment into a positive in some way in our soul and our heart.