However, the Ohr haZarua (shu”t 128) this concept specifically applies to Kiddhushin but does not apply to other Mitzvos such as Shechitah or separation of Chalah. Rabbi Sholomo Kruger (in his sefer Sta”m kuntrus Peret and Ollalos) says this concept applies to Kiddushin and Shabbos, but not other Mitzvos.
Rabbi Kruger explains the reason for this distinction as follows:
We find that HaShem is very specific in rewarding good deeds.as we find in the Bava Metziah (86b). The Gemara tells us that Avrohom Avinu was rewarded for each activity that he performed as he worked to host the angels who came to visit him. In response to each action he performed on his own, HaShem performed a corresponding favor directly on behalf of the Jewish people, the descendants of Avrohom. However, for each action which Avrohom delegated to others to do, HaShem arranged that it be done “on the part of a messenger” on behalf of the Jewish people. For example, Avrohom himself ran to prepare meat (Bereshis 18:7), and as a reward, HaShem later directly sent the pheasant birds for the Jews (BeMidbar 11:31). Avrohom brought butter and bread (Bereshis 18:8), and the Jews were provided with heavenly manna directly from HaShem (Shemos 16:4). However, he sent someone to bring water to the guests (Bereshis 18:4) when he said, “Let some water be brought, please, and wash your feet, and recline beneath the tree.” As a result, the Jews were furnished with water only via a messenger, through Moshe, as we find in Shemos 17:6.
(Rabbi Moshe Feinstein notes that we might wonder why the actions of Avrohom Avinu are considered deficient and somewhat lacking in his appointing a messenger to take care of bringing water to the guests. Avrohom was very diligent in all his actions, and he certainly intended to see that everything was done efficiently. In fact, after he brought the cattle, he gave them to "the lad" to prepare them. Rashi identifies this lad as Yishmael, and he explains that Avrohom handed them to Yishmael in order to train him in the mitzvah of accommodating guests. Here, too, in reference to the water, we can be assured that Avrohom intended to train this messenger in the mitzvah. Why, then, is this gesture indicative of some degree of a failure on his part? We see, though, that in order to fulfill the mitzvah of training others to do Mitzvos, the best method is for a son or a student to witness his father or mentor himself being involved in the mitzvah. It is never enough for the parent or teacher to merely lecture or instruct others on what they should do. Therefore, there was no excuse for Avrohom to delegate these actions which were necessary to accommodate the guests. The best way to educate is to serve as a role model, a consistent and steady image of commitment and loyalty, one which is there to be observed and to be emulated.)
With this lesson from Avrohom Avinu Rabbi Kruger explains regarding this concept that a Mitzvah is better if done by oneself as applying only to Kiddushin and to Shabbos. The laws of Shabbos were explained together with the falling double of the Mahn on Erev Shabbos and none on Shabbos. Thus, the laws of Shabbos were directly transmitted to the descendants of Avrohom. Further, the Mitzvah of Kiddushin is connected to the Mitzvah of procreation which HaShem gave directly to Noach. Thus, these two Mitzvos are considered given directly to the Jewish people and the rest of the Mitzvos were given by Moshe, the agent of HaShem.
The Yad haMelch provided a different explanation of the distinction that a Mitzvah is better if done by oneself as applying only to Kiddushin and to Shabbos. If the action was done by an agent, the possibility is that the individual would completely miss involvement in the Mitzvah.
Regarding the preparation for Shabbos and its resultant burdens is a separate portion of the concept of honoring Shabbos. Only the person burdened and doing the preparation participants in the fulfillment of this aspect of honoring Shabbos.
Regarding Kiddushin, if a person arranges for Kiddushin through an agent and uses money from his servant he would not be personally participating at all in this Mitzvah.
Therefore, the concept of doing these two Mitzvos by oneself rather than through an agent is different than for all the other Mitzvos.
