At weddings when the chosson and kallah are walking to the chuppah everybody stands as well. Rav Soloveitchik pointed out that people think it's because a chosson is like a king [חתן דומה למלך] and you stand for a king [and presumably the king is marrying a queen who also deserves honor]. But he explained that they are not yet chosson and kallah. That only happens AFTER he gives her the ring in front of the witnesses. The true reason we stand is because they are on their way to do the mitzva of getting married. [I add that you should look at the end of the sefer Bikkurei Yaakov on Hilchos Succah by Rav Yaakov Etlinger and you will see him sing a different niggun.]
So the poskim wonder why we don't stand for a tzedaka collecter. He is also doing a mitzva?
Maybe we should. But we don't. At best we get up ... and run in the other direction. In the Tshuvos Vihanhagos [Vol. 4 Simman 218] he suggests that we only stand up for mitzvas that take place from time to time, as the gemara says חביבה מצוה בשעתה. But tzedaka collecting is a constant occurrence and therefore we are not obligated to stand before the collectors. But if you do - blessings should come on your head.
