There's a whole plethora of reasons for this. The biggest reason, however, is medical.
My foreskin is WAY too tight. It doesn't retract at all if I'm erect. I can retract it if I'm soft, but it's still tight. I've been working the foreskin for several years, trying to get it to stretch, but nothing's worked and of the 4 urologists I've gone to, all have said that there's nothing surgical they can do to it that will make it retract.
I'm fine with those diagnoses. I want it gone. Sex has never been really enjoyable (as a top) because of it, and I honestly want to experience sex that doesn't involve either no feeling whatsoever (since the foreskin doesn't retract when I'm erect), or a searing, burning paper-cut type of pain if the foreskin actually DOES get pulled back during a bump & grind session.
<--- That's me when it (my foreskin) DOES get retracted during sex.
For the TMI portion: I'm not a big fan of being a bottom, but in order for past relationships to stay on track sexually, I did what I had to do in the bedroom in order to ensure my partner was satisified. Being a top is what I prefer, but as stated before, it's never been enjoyable and many times I've had to push through the intense pain and discomfort in order to make sure my partner was pleasured. I want to experience GOOD sex, that is free from pain and discomfort; I believe this is the best way to accomplish this goal.