That one can merit two portions in Eden, or two portions in Gehennim, is easy to understand. However, why does one's merit in Eden cause one's friend to forego? Can't both one and one's friend be meritorious, or both culpable?
H. David Levine, Roanoke, VA; USA
The answer to your question, in short, is that the person's friend is not meritorious. The Tzadik is receiving the portion of Gan Eden that his friend would have received had he done the Mitzvos. Similarly, if one receives his friend's portion in Gehinnom, his friend is not culpable. The explanation for this is as follows.
Every person has the ability to do Mitzvos. When they do not do a certain Mitzvah, they make it much harder for others to do that Mitzvah, as they create an environment in which the norm is not to do that Mitzvah. Accordingly, the Tzadik who does the Mitzvah anyway receives the portion in Gan Eden that this person (i.e. sinner) would have received had he done the Mitzvah. This is because his refraining from doing the Mitzvah made it harder for him to perform it. The same applies regarding Gehinnom. The sinner is getting punished not just for his sin, but he takes his friend's portion in Gehinnom as well as his actions caused a difficult environment for his friend to perform this Mitzvah. (See the explanation of the Yedei Moshe on the beginning of Shemos Rabah in Parshas Ki Sisa, who gives a similar explanation.)
All the best,
Yaakov Montrose
So if everyone does teshuva, all of a sudden the Tzadikim who traded their portions in Gehenom for an extra portion in Gan Eden will have no one to trade with, and find themselves with portions in Gehenom?
Obviously Tzaddikim will not have a portion in Gehinnom, as they never earnedthat portion in Gehinnom since they never did any sins. The meaning of "everyone has a portion in Gehinnom" is that everyone has a potential portion in Gehinnom if they exercise their free will to sin. The Rasha earns his Tzadik friend's
potential portion in Gehinnom as well if he makes it harder for the Tzadik to do the Mitzvos by creating an environment of sin.
All the best,
Yaakov Montrose