There's an ongoing debate on whether kinks and fetishes are nature vs. nurture, but to me, asking "Why are you into feedism?" is like asking "Why are you gay?"
In my experience, there's no smoking gun that I can point to that triggered my attraction to weight gain. There were lots of moments where I realized more and more that I was attracted to people getting fat, but they were no different than moments where I discovered my attraction to typical things, like breasts or butt or thighs.
Don't get me wrong, I've wondered for a long time what caused my unusual attraction, because surely something caused me to be this way, right?
When I told my first long term SO about this, she kept asking me to explain why I felt this way, why fat was attractive to me. And I remember asking her why she found my *ahem*, uh, 'manhood' attractive, and she struggled to explain why exactly that part turned her on. And how could she? How do you explain an attraction – not evolutionarily, not culturally, but how it feels to be attracted to something? Why you feel attracted to something?
I guess what I'm trying to say is that it's okay to not have an explanation as to why we are into feedism, and that our attractions are just as valid and justified as those that are typical.
Oh, and that episode of Totally Spies! where Clover gets addicted to cookies served as something of an awakening for me