Why do some verbs take 'avoir' as the auxiliary whilst others take 'être'?

The vast majority of verbs use 'avoir' as an auxiliary. However a few take être. These verbs are mostly verbs which communicate a state or motion. Just remember MRS VANDERTRAMP Monter Retourner Sortir Venir Arriver Naître Descendre Entrer Rester Tomber Aller Mourir Partir