[Bungo Baggins]

I think Tolkien's deep-rooted Catholicism was the reason he saw great pride motivated by power as the beginning of one's downfall. He also knew enough about myths and religions to know pride is always evident before one falls: In Christianity, pride is often regarded as the first step to the fall and rebellion against God. In Buddhism, grasping after the self and the sense of ego is the chief of all cravings and the deepest root of ignorance. In the Indic religions pride, like ignorance, is a fetter that binds humans to the wheel of rebirth.

I think he also realized motives were key in whetherour pride corrupts us or not. Saruman, Sauron and Morgoth all were power hungry and possessed great pride and all paid the price. Aragorn had some pride for sure, but he was motivated by helping Frodo in his quest to destroy the ring.