Question

What makes it so fascinating for one person to pee their undies or a diaper?

What makes it so fascinating for one person to poop their undies or a diaper?

What triggered that idea for one person to want to wear diapers again?

I suppose that depends (no pun intended). I guess there might be a difference between someone who is into ageplay and someone who is “just” a DL. And even within the same group people might have wildly different reasons. You could just as easily ask a person with a foot fetish why they find feet so fascinating. Think about yourself. What are the reasons for you liking the things you like? Do you think other people who like the same things like them for the same reasons?