Before Pesach one must destroy all of the chametz in his possession. What happens if you left on a trip and forgot some chametz at home? The Rambam says that if you are traveling for a mitzvah then if you are unable to both return and destroy the chametz and also do the mitzva, then it is sufficient to do mental Bitul (thinking of it as the dust, abandoning any interest in ownership or use). This also applies when traveling to a meal of a Mitzva, such as a wedding feast. Such a feast is also considered a type of Mitzva. Why should this be? What is the Mitzvah of eating at a meal celebrating a Mitzvah?
Devarim 28:47 ascribes the curses, including exile, to not doing the Mitzvot with joy. In hilchot Lulav 8:15, he explains this pasuk as the basis of the Simchat Beit hashoeva. The joy in doing Mitzvot is itself an important form of divine service. And its absence is worthy of punishment, even if the person is checking off all of the requirements. In a sense this is an extension of the intuition that a person who happily lives an ethical life is greater than one who does so begrudgingly. Morality is not only about what you do, but also what kind of person you are. Serving God is the same way.
This ideal of serving God with joy develops from the last two chapters of Hilchot Teshuva. In 9:1 he explains the harm caused by the absence of joy. And in chapter 10 he explains the ultimate root of this joy – love of God. Insofar as a person experiences the Mitzvot as good in their own right, they will enjoy doing them, and rejoice in the privilege of participation.
This is the importance of a Mitzvah feast. Doing Mitzvot is not enough, rather we must celebrate the doing of Mitzvot. Even though the full rejoicing is limited to people who serve God from love, all of us have an obligation to experience the joy to the degree we are able. A mitzva feast is a vehicle of this. Since eating and drinking and partying make us happy, the Torah encourages us to use them instrumentally for a greater purpose. Celebration is wonderful, but the key question is what we choose to celebrate. That choice both reflects our true values, and also the values which we aspire to.
Of course this has implications for chinuch as well. One of our key jobs as parents or teachers is to provide our children with models of the joy of living the life of Mitzvot and ethics; and with opportunities to experience it at their level.
The next part of the halacha is also worth mentioning. If a person is traveling on a rescue mission, then even if there is time to go back one doesn’t do so, rather they should stay focused on their mission and suffice with negating the Chametz in their heart. As is generally the case, protecting life is taken more seriously then a Mitzvah.