It's a simple case of "the grass is greener on the other side;" all ~95% of men in America can do is imagine what a foreskin must be like (aside from seeing pictures in biology class and the rare Euro porn), so one of two things happens:

They're repulsed by it and say "I'm glad I was cut" (most common), or they begin to develope a fixation on it, and some even begin to hate their parents for having it done to them (the NOCIRC movement consists almost entirely of men who are bitter about their circumcision).

They want to be with uncut guys so they can see what it looks like, what it feels like, etc. I've had older gay men hit on me simply because i'm uncut, and they started to ask me all kinds of off-the-wall questions like:

And one guy actually demanded to try docking with me, though my foreskin isn't long or loose enough to accommodate a second penis.