Commentary for Deuteronomy 32:50

כאשר מת אהרן אחיך [AND DIE] … AS AARON THY BROTHER DIED — i.e., and die by that death which you witnessed and for which you longed. For Moses stripped Aaron (before the latter’s death) of his first (i.e., the upper) garment, similarly the second, and similarly the third, and attired Elazar with them, and thus he (Aaron) beheld his son in his new dignity. Moses then said to him, “Aaron, my brother, ascend the bier”, and he ascended; — “Stretch out your hand”, and he stretched it out; — “Stretch out your leg”, and he stretched it out; “Close your eyes”, and he closed them; — “Close your mouth”, and he closed it, and thus did he pass away! Thereupon Moses said, “Happy is he who will die a death like this!” (Sifrei Devarim 339:3; cf. Rashi on Numbers 20:26).

ומות בהר, accept your death willingly so that it will be an atonement for your trespasses

ומת בהר, "and die on the mountain." G'd wanted Moses to be willing to die. G'd does not remove the souls of the righteous from their bodies unless they agree. G'd added והאסף אל עמיך, "and be gathered in to your people," this is an allusion to what the Talmud says in Ketuvot 104 that the righteous in the hereafter come out to welcome the righteous who is about to join them. G'd told Moses he was going to be met by a reception committee.