Комментарий о цитате к Шмот 26:5

And pleasant: This is calmness and ease. And he said with this command that when you are in front of a man of great standing, make yourself light towards him and serve him and stand in his presence if he wants and do not seek honor for yourself from him. But when you are with black hair - meaning to say one of young years - do not do this; but seek honor for yourself from him and do not be playful and do not be jocular with him. Afterwards he said, "You should not think that that which I have warned you from being jocular with someone of young years obligates that you should greet him with rage and with a choleric face. This is not the intention. Rather you need to greet every person - small or big, freeman or slave, every man of the human species - with joy." And this is more than that which Shammai said (Avot 1:15), "with a pleasant countenance." [The translator said it appears to me from the words of the teacher (Rambam) that he explains the word mekabel (literally, receive) from hakbelat panim (being across from the face) which is as if he is across. This is derived from (Exodus 26:5) "the loops across (makbilot)," which he translated from the Aramaic as across or facing. And also in Arabic one says about this matter an expression of meeting and being opposite - one says x met me happily or angrily. And with this type of expression did the teacher explain mekabel here - and know it.]

AND RECEIVE [Heb. mekabel] EVERY MAN GLADLY. Rav: stand with joy. Mekabel here is to be understood like makbil. The medieval translator of Rambam writes on this mishna: it seems he understands the word mekabel here as if it were makbil, as in makbilot halula’ot (Exodus 26:5), based on the use of the cognate root in Aramaic, where the root is used to translate¹¹³In the Targumim. “opposite” or “in the presence of.” The Arabic language uses similar expressions: in speaking of meeting or standing opposite somebody one says “so-and-so met me with joy or anger.” The master¹¹⁴Rambam. explains mekabel this way here.
I do not know why he did not explain this point in the mishna at 1:15 above: hevei mekabel et kol ha’adam besever panim yafot.