I did not want to interrupt the flow of the previous post with this point, so I decided to add it here. Those of you who (like myself) are from the YU world are probably perplexed by that Rashi in Sukkah 25a that says there is no chiyuv for a mourner to be sad. The Rav (see Shiurim l'Zecher Aba Mari vol 2 - B'Inyanei Aveilus) saw aveilus as consisting of two parts: 1) the ma'aseh mitzvah, the to-do behavioral component of not washing, not wearing shoes, etc., and 2) the kiyum mitzvah, which is a chovas he'lev to feel that sense of loss and mourning. If anything, the Rav placed more stress on the kiyum mitzvah as the driver of aveilus. The reason simchas haregel pushes off aveilus is not because not wearing shoes or taking a shower is inherently antithetical to the observance of the regel, but because the feeling of simchas yom tov is antithetical to the kiyum mitzvah of grieving. (The Rav had many proofs to this idea of a kiyum she-b'lev.) So what do you do with the Rashi that says "aino chayav l'hitzta'er"? Doesn't that imply (as R' Shlomo Fischer suggests) that there is no mitzvah per se to be sad -- that there is no kiyum she-b'lev component to aveilus?
I think the answer is that Rashi needs to be taken in the specific context of explaining oseik b'mitzvah patur min hamitzvah. Of course there is a *kiyum* mitzvah in a mourner grieving, but there is no *chiyuv* to be sad in the same way that there is a chiyuv to take action and do some other mitzvah. By definition, a kiyum is a result, an effect -- not a cause. Oseik b'miztvah deals with causes, i.e. can a new mitzvah serve as a cause to change behavior when there already exists a cause that forces one to engage in some other action.
Of course, if I am correct that undermines R' Shlomo Fischer's point to some degree, but I'm just trying to defend the Rav's torah. (After writing this I see R' Reichman asks the same question in his Reshimos Shiurim on Sukkah, but b'mechilas kvodo I like my answer better than his, ayen sham : )