I asked my question once again at the Daf Yomi shiur that I give every Thursday night in Congregation Ahavas Achim B'nai Jacob and David in West Orange, NJ. (There is a rotation so a different person gives the shiur each night of the week.) I argued that the mitzvah of אבילות should be analogous to the mitzvah of שמחה, joy, on Yom Tov, or the mitzvah of כוונה, intent, when one prays.
In each of these mitzvot one has a מעשה המצוה, an action one does to fulfill the mitzvah, and a separate קיום המצוה, what causes the mitzvah to actually be fulfilled. For example, by the joy of Yom Tov, the Rambam (הלכות שביתת יום טוב פרק ו' הלכה יח) lists many actions for this mitzvah including eating meat and drinking wine, buying new clothes or jewelry, or giving sweets to children. However, each of these actions is supposed to lead to a קיום המצוה, the feeling of happiness on the Yom Tov. The actions are a means to this end but the mitzvah is to fell joy NOT to eat meat. A vegetarian, for example, who does not enjoy meat would not have a mitzvah to eat meat on Yom Tov.
Likewise, in prayer, the reciting of the words is merely a מעשה המצוה. The קיום המצוה is in the כוונה שבלב, the feeling engendered in one's heart. As the Gemara says often, while by other mitzvot like קריאת שמע, there might be a dispute whether the mitzvah requires כוונה, by תפילה everyone agrees that without the כוונה שבלב, the prayer is meaningless. The קיום המצוה is in the כוונה.
I always thought that אבילות, mourning, worked the same way. The tearing of קריעה, the accepting of consolation, ניחום אבלים, the sitting of שבע near the floor, the abstaining from bathing etc. were merely the מעשה המצוה. While the קיום המצוה was in the feeling of sadness and distress.
In my shiur, some of the participants argued that I might be wrong. Perhaps the actual mitzvah by אבילות are these actions. One fulfills אבילות even without the feeling of distress and sadness. This would explain the distinction in our Gemara. The sadness of the אבל is not considered to be the mitzvah and therefore the אבל is obligated in קריאת שמע.
The better way to understand אבילות is that these actions are designed to give us the space and time to mourn, to feel the loss. But this feeling is completely subjective and cannot be legislated by the Torah. The Torah in its wisdom understands that אבילות is a profoundly personal experience. The actions are there to help each person get through this experience but ultimately each experience is up to the individual.