Ah, the age-old question. I've never actually been asked this, because most of the people who know me in RL don't know I like it, and I've only ever tried to puzzle it out in my own mind. But here's my personal answer to the "Why do you like it?" query:
I have no fracking clue. I had a dream about it once when I was eleven, and I never forgot it. It just stuck in my mind, and I wasn't disgusted or freaked out by the idea. I was a shy, often-teased kid, so I didn't tell anyone that I liked it for years. I thought I was the weirdest person ever and the only one who liked Mpreg (I didn't even know what that term meant until I discovered Internet at age fourteen.) in all the world. Needless to say, I never even wrote stories about it. It was my deepest, darkest secret.
So, then came the age of Internets. I was a Harry Potter fan by the time I started my sophomore year of high school, so I found Harry Potter Mpreg pretty darn fast online. I was SHOCKED. I couldn't believe someone else liked MPreg, too! The knowledge that I wasn't a complete freak and somebody else saw the appeal of it as well was so liberating. I started writing a sixteen-chapter HP Mpreg that I never posted online, but it felt great just to write it all out and see the story come together. I'm happy to say that my writing's definitely improved since then, but I am still proud of that story. It was my very first Mpreg ever, and I enjoyed every second of it.
So, for a good ten years, I was a total HP Mpreg fan. I wrote it, read it, and beta-ed it. I read mediocre, horrible, pretty good and absolutely wonderful Mpreg. It helped me realize that there was absolutely nothing wrong with having an affinity for something that other people considered "gross" or "weird." I've heard every argument against it, and they used to bother me, but now I just roll my eyes and think, "Really? You're entitled to your opinion, of course, but it's NOT REAL. Calm the fuck down." Because some people react so adversely to it, I wonder if they honestly think all MPreg fans are mad scientists performing Human Centipede-like experiments in their basements, transplanting wombs into male victims or something. It's not a real and present threat, people! We're just using our imaginations!
I've said my goodbyes to the HP fandom now, and I liked some Supernatural MPreg that I found when I was watching the first two seasons on DVD and I realized that this was why all those people were rooting for two brothers to get together (heh.) I wrote some of the best Mpreg I've ever written for the Merlin fandom back in 2010, but my love of that show was quickly eclipsed by Supernatural. Still, I love the stuff I wrote for Merlin and it was so much fun.
Also....I suppose my love of Mpreg is psychological. I don't want to have kids myself. I don't want to get pregnant, I don't want to go through childbirth, and I definitely am nowhere near ready for the financial, physical and emotional demands of motherhood. However, I love the idea of MEN doing all that, especially if a man loved me enough to carry our kid himself. Wishful thinking ahoy! But that kind of image is, for me, simultaneously erotic, deeply moving, and makes me feel super-protective. Seriously, if in some alternate universe or whatnot I met a guy who could and did consent to carry our child himself, I would be the ultimate Mama Bear. Touch my baby-daddy and DIE, motherfucker! I would be a self-appointed bodyguard for him. And he'd probably get disgruntled and tell me that I didn't have to be so overprotective, and I'd probably have to tone it down just so I didn't cause too much trouble, but it would come from a good place! And nobody could say that I didn't have maternal instincts, could they?
Why, yes, I have spent a lot of time thinking about this. :)
Finally, this is "why I like it" from a feminist POV: Everyone romanticizes how loving and selfless and wonderful mothers are, for sacrificing their bodily autonomy and comfort and going through all that incredible pain to bring another person into the world. Well, how awesome would it be if a guy did that? I know there are a million valid reasons why most men would never want to trade places in the whole childbearing arena, but I personally would have the deepest respect for any guy who took that on. It would also blow gender stereotypes to smithereens and maybe FINALLY we could get some improved perspective on what is "women's work" and what is "men's work."
So...yep. That is Why I Love Mpreg in a nutshell. What are YOUR reasons?