I've been contemplating this for several decades. Back in the 1990's, Yahoo groups began to form, dedicated to people with giantess and vore fantasies/fetishes. Until then, I'm pretty sure all of us just thought we were nuts and kept it a secret.
After seeing tens of thousands of members join, there was a gleam of hope that we are not alone...there are others who share our fantasy, and that helped to ease some of the embarrassment and shame surrounding this fantasy/fetish.
I've exhausted many possibilities as to the genesis of this fantasy....conditioning, curiosity, comfort, a plethora of others. I'm still not entirely convinced there isn't a potentially spiritual factor at play - something along the lines of samsara, or reincarnation. If I had been eaten in a past life, perhaps that is why vore resonates so much with me.
Truthfully, I think if you were able to figure out why all of us have the desire we do, there'd be a cornucopia of different reasons. Just as we are all wired in vastly different ways that make our fingerprint in the universe truly unique, we were led to our desires through different paths.
Of course I hope to be proven wrong, and find out there is a much simpler, more quantifiable reasoning behind it, but after decades of contemplation, I am still at a loss. Silver lining: the not knowing makes it all the more mysterious! We weren't meant to be able to explain EVERYTHING in the universe. How boring would that be?