What is the reason "Al Tis'hader Lifnei Melech"?
Rashi: Do not show your honor and be haughty in front of the king, or stand if someone greater than you, for "it is better that they tell you to ascend" (7), than for you to enter without permission and they will tell you 'leave'!
Malbim: Do not show any honor [for yourself] in front of the king, for your honor is Batel to his honor, which has no limit. When you show honor, as if also your honor is considerable, you belittle the king's honor. This is like David said "v'Sachakti Lifnei Hashem" (Shmuel II, 6:21). He danced and was jovial like a commoner in front of Hashem. The Yerushalmi (Brachos 1:6) says, he said "Urah Kevodi Urah ha'Nevel v'Chinor A'irah Shachar" - let my honor be [Batel, and I will be] woken due to the honor of my Creator. My honor has no significance in front of my Creator's honor.
Why should you not stand in the place of Gedolim?
Mo'ed Katan 28b: One who does not aggrandize himself, others will elevate him.
Refer to 25:6:1:1.
Malbim (6-7): It is for the same reason as not honoring yourself in front of the king (your honor is Batel to his). If you stand in the place of Gedolim, they will lower you in front of the Nediv, that you thought that you are at his level. This will be a disgrace for you! Rather, stand in a place below your level, and not in a place higher than it. This will cause them to tell you to ascend.
