Bukhari

Hadith Commentary:
(1)
From these ahadith, it is understood how much love the Messenger of Allah (sallallahu alayhi wa sallam) had in his heart for people purely on the basis of humanity, regardless of whether they were Muslim or Jewish; he would stand upon seeing a funeral procession.
Several reasons have been mentioned regarding this:
In these very ahadith, it is stated that in matters of life, Muslims and non-Muslims are all equal.
Everyone is confronted with life and death.
The Messenger of Allah (sallallahu alayhi wa sallam) would stand out of respect for humanity.
Some narrations indicate that standing for the funeral was due to reverence for Allah’s command and the greatness of death, and that one should welcome it by standing.
Death is something that causes anxiety, no matter whose it is.
Upon seeing it, one should feel apprehension and, being overawed by it, should stand.
(2)
From the narration in Sunan an-Nasa’i and others, it is understood that the Messenger of Allah (sallallahu alayhi wa sallam) would stand out of respect for the angels (Sunan an-Nasa’i, Al-Jana’iz, Hadith: 1931)
and would also command others to stand.
In any case, this standing was practiced earlier, but later it was abandoned, as we have already explained in detail.
And Allah knows best.