Abaye explains the Gemora as follows: In the case of a vow, one says, “The pleasure of the sukkah should be forbidden upon me.” (Since the vow is upon the object, not the person, it is effective.)  However, in the case of an oath, one says, “I swear that I will not derive any pleasure from the sukkah.” (Since the oath is upon the person and the mitzvah is upon the person, they are contradicting each other and the oath cannot take effect.)

Rava asks on Abaye’s language: Were mitzvos given to derive benefit from them? (He should still be able to sit in the sukkah?)

Rather, Rava explains: In the case of a vow, one says, “The sitting in the sukkah should be forbidden upon me.” However, in the case of an oath, one says, “I swear that I will not sit in the sukkah.”

The Reshash asks on Rava’s question: Although the mitzvos were not given for the purpose of deriving benefit from them, it still should be forbidden to sit in the sukkah in this situation? This is because he is enjoying the incidental pleasure of the shade!

He answers that if it is shade that the person desires, he could stay inside the house. The reason he chose to sit in a sukkah is solely because of the mitzvah. It is therefore not regarded as if he is befitting from the shade of the sukkah.

The Machaneh Efraim is not satisfied with this answer. He asks that the bottom line is that he is not dwelling in his house; he is inside the sukkah and enjoying its shade! Why should this be permitted?

The Oneg Yom Tov answers: We are only concerned with the physical pleasure when the benefit is purely incidental and not related to the mitzvah. However, the mitzvah of sukkah is different. The Torah commands us to dwell in the sukkah in the same manner that one would live in his house. The enjoyment of sitting in a sukkah is not regarded as an incidental benefit; this is the fulfillment of the mitzvah. And since mitzvos were not given for the purpose of deriving pleasure from them, the (direct) benefit that one receives as he is fulfilling the mitzvah is not regarded as a benefit, and is therefore permitted.