As related by Michael Broyde
Fasting on Yom Kippur also fits into this category of mitzvos which have no temptation attached to them. For on Yom Kippur people are entirely engrossed in the holiness of the day and forget or are too awed to think about eating and drinking. Nevertheless, this mitzvah of fasting also receives the great reward mentioned above.
The lesson of this midrash is clear. The impact of our deeds is much greater than we realize. Adam’s one sin caused death to come into the world and affected all the generations to the end of days. We see the impact of his sin constantly, since no one can escape from death.
But if this results from a single sin, imagine how many times greater is the impact of refraining from sin, or doing a mitzva. Our Sages tell us that the influence of a good middah is five hundred times greater than that of a bad middah. If the results of such a sin were felt by every one of us for so many generations, the reward for a positive mitzvah is also felt universally. In the case of Rabbi Yisrael Hagar the impact of his mitzvah was so great that it caused a person to begin observing mitzvos because of it. Even though we may not see such a dramatic impact, it nevertheless has an effect in one way or another. If we realize this and take great care not to sin, we will reap the great reward that awaits us and those who will come after us.
Our Sages say, “G-d wanted to give Israel merit, therefore He multiplied for them the Torah and the mitzvos.”2 Since we have learned from Adam the impact of one single mitzva, we can now appreciate why G-d wanted to multiply this reward. G-d in His great kindness wanted to benefit us, so He gave us many opportunities to gain that reward.