The motivation is this interesting story... ' After a major military victory, the triumphant military generals were paraded through the streets to the roars of the masses. The ceremonial procession could span the course of a day with the military leader riding in a chariot drawn by four horses. There was not a more coveted honor. The general was idolized, viewed as divine by his troops and the public alike. But riding in the same chariot, standing just behind the worshipped general, was a slave. The slave’s sole responsibility for the entirety of the procession was to whisper in the general’s ear continuously, “Respice post te. Hominem te esse memento. Memento mori!”

“Look behind. Remember thou art mortal. Remember you must die!”

The slave served to remind the victor at the peak of glory, this god-like adoration would soon end, while the truth of his mortality remained. '

That story is given here in the section titled "Romans": https://dailystoic.com/history-of-memento-mori/