GEMARA KIDDUSHIN - PEREK BET

We  will be continuing this series of shiurim with  Perek Bet.  Sources for next week's shiur:

1.  Kiddushin  41a "Ha-ish...armelu," Shabbat  119a  "Rav Safra...kamei."

2.  Rashi  Kiddushin s.v. mitzva bo, Tosafot Ri  Ha-zaken mitzva bo... shaliach."

3. Rambam Hilkhot Ishut 3:19, Hilkhot Shabbat 30:6.

4. Shulchan Arukh Orach Chayim 250:1, Magen Avraham ibid.
                           
                           
Questions:

1.   Regarding  what  specific  cases  does  the   gemara explicitly prefer personal involvement?

2.  Should  this  preference be  applied  to  mitzvot  in general?

3.  The  Yad  David  limits the  preference  of  personal involvement  to  those cases mentioned explicitly.   What might be the reason for this?