kind of going off of what >>3607
was talking about, I like the kind of head-space it creates. One of the things I like about being diapered/little/regressed the most is the sense of being totally carefree/unconcerned in general, and that extends to bathroom activities as well. Like, imagine being a little kid, around toddler age, and you are probably at least partially aware of your need to pee/poop, but you havn't started potty training yet. When you have to go, you just stop playing or running around for a moment and use your diaper, then go back to what you were doing without a care in the world. Not only is there no expectation for you to use the toilet, but the idea of peeing or pooping anywhere but your diaper has just never crossed your mind. Whenever you have to go, you just go in your diaper without a second thought. Where else would you even go? Thats just the way the world works for you.
Again, like >>3607
said, the humiliation is also a part of it for me. I dont think I would call it that though, more like "shyness" or "embarrassment". When you pee in a diaper, there usually isn't any super obvious sign to tell others (or yourself, to some extent) that you peed yourself. Sure, some diapers have some sort of wetness indicator, and eventually the swelling and/or discoloration would make it fairly obvious, but if you are wearing pants, that is mostly hidden. And while there might be some smell from pee, it usually isnt all that strong, and if it is noticeable at all, it probably wouldnt be noticeable right away. You could be playing with legos or something in the same room as a big/cg (or even right next to them, like playing a board game or something) and wet yourself and its very possible that they would have no idea. They know that over the course of the day you are wetting yourself, yes, but they don't know the exact moment it happens. With poop on the other hand, it is pretty much *instantly* obvious to everyone nearby that something is happening in your diaper. First of all, they would probably notice you pushing, but even if they dont (either because they weren't looking at that moment or because you were able to be really subtle/sneaky about it) then within a very short period of time they will probably smell you too. I think that might be one of my favorite things about it, because you cant hide the smell really. All you can do is blush and wait for them to say something. As I said, I wouldnt say I like feeling humiliation, but I like feeling shy, blush-y, and bashful about it. Like imagine being simultaneous shy/bashful/embarrassed cause you pooped yourself and "Ewww, poop! pee-yewww!" but it also being treated as a silly, giggly thing at the same time.
Another side is the lack of control. If you use a laxative/suppository/enema/etc, you can basically remove your ability to control it, and that sense of helplessness does things for some people who like messing as well. I sometimes like to do this and try to hold it as long as I can, like genuinely try my best to not have an accident. This also compounds with some of the other things mentioned above, especially if it ends up being noisy as well. The noise/smell takes something that we normally see as a private activity and makes it much less private, because others are now being made aware of what is happening down there more or less in real time, and theres not much if anything you can do about it. A big/cg can watch you squirm and become increasingly more visually desperate, starting out with just acting a little weird to having moments where you hold really still while you have to try really hard to hold it in, then eventually progressing to the point where you are just constantly dancing and squirming to keep from having an accident, then you get to the point where you are practically shaking with the effort and finally just physically not being able to help yourself anymore and totally losing control in your diaper.
Personally, I dont really like the mess part of it. It IS gross, but personally I am mostly able to put that part of it on the back burner of my mind and just not worry about it in the moment. For the most part, as long as everything stays in the diaper, and I don't feel like there is any risk that anything will get out, I am fine. There are also different kinds of mess, some of which are less "messy" than others. I personally prefer a mostly firm mess, because it doesn't get everywhere, doesnt stick to your skin, and is pretty easy to clean up, but at the same time is soft enough that its not too difficult physically to get it out into your diaper. I also usually clean up by taking a shower. I just wipe off the worst of it with the diaper itself, then spray anything that didnt come off with the shower head. Unless you end up with a "messy" mess, there usually isn't actually all that much to get off. The big thing though is that I'm not actually touching anything with my hands.