that Mount Doom was more than just a source of heat for Sauron's forge, but was
in fact a concentrated source for Morgoth's taint. In Morgoth's Ring
Tolkien explains that Morgoth invested his power in Middle-earth much as Sauron
invested his power in the Ring. But Morgoth is so much greater than
Sauron that all of Middle-earth became Morgoth's ring. Morgoth's taint
was especially concentrated some areas (such as gold) and less so in other
areas (such as bodies of water, or mithril). Although Sauron invested his
power in his ring, what really made his ring powerful was that it allowed him
to tap into the power of Morgoth's taint.
All of this is a long way of saying that perhaps Mount Doom is a place where
Morgoth's taint is concentrated, and therefore is in large part the source of
the Ring's power.
Also this means it is easier for Tolkien to explain why the Ring did not react
more strongly to Sauron's presence across the river in Mirkwood during the
several centuries Gollum had the
Ring.
“I dislike Allegory - the conscious and intentional allegory - yet any attempt to explain the purport of myth or fairytale must use allegorical language. (And, of course, the more 'life' a story has the more readily will it be susceptible of allegorical interpretations: while the better a deliberate allegory is made the more nearly will it be acceptable just as a story.)” (From Tolkien Letter # 131.)
