@sexypussycat: I've been a diaper-lover all my life, and an adult-toddler diaperboy since I reached the age of majority. I took no offense from your reasonable, sincere, and welcome inquiry. Your question has been asked by others, though in a more direct and graphic form, and as my equally direct and graphic answers to those probing, searching questions have been excised twice now, I'm going to take a different tack in answering your sensible question.
First, here's a thumbnail sketch I wrote that is heavily allusive and that probably raises more questions than it answers:
http://www.zity.biz/index.php?mx=blog;ox=showitem;id=632#page2280
Second, I will write from my own perspective that I find commonality in imprinting across similar fetishes emanating from childhood (but having nothing whatsoever do with minors), such as enemas, suppositories, rectal-temperature-taking, and even pediatric visits. I've written of these commonalities elsewhere, so I won't reinvent the wheel. You may look under my or any member's profile and read my or their posts and other contributions; I encourage you to do that.
Here are links to several threads in which I've posted that might prove elucidative:
http://www.zity.biz/index.php?mx=forum;ox=display;topic=71672.msg508261#msg508261
http://www.zity.biz/index.php?mx=forum;ox=display;topic=70476.msg505002#msg505002
Third, I will write that each adult-diaper wearer's experiences in childhood and adulthood are unique. I see adult-baby and diaper-lover and related activities, behaviors, and needs as lying along a spectrum, with certain trends and tendencies marked as signposts along the way (e.g. many, but not all, adult babies refrain from sexual activity during diaper-change). My point in this regard is that there are no rules, save those obvious proscriptions dictated by the law and morality.
So why do I wear diapers as a middle-aged professional man? Well, I suffered severe, prolonged, and sustained child abuse in the household. This caused not only classic Freudian complexes but also significant organic brain and nervous-system damage and deep emotional wounding during early development that left me unable to toilet-train until I went off to college at sixteen. I remain moderately incontinent to this day, and so diapers make the most sense for me and provide me with necessary protection. I was lovingly and intensively diapered by my Aunt in Pampers from birth through nearly the age of four, and she imprinted me with my diaper fetish. Pampers had a powerful and unique scent back then, and they had a silky plastic backsheet that could not be beat. My Mom hated changing my wet, soiled cloth diapers and plastic pants, but my Aunt loved changing my wet, messy Pampers. This whipsawing opposition left me with a love of Pampers as a transitional object -- a love that persists to this day. However, I now also embrace wearing, using, rinsing, soaking, and laundering my custom-made adult cloth diapers and plastic pants as part of a regimen of therapeutic identifactive reparenting that has been remarkably successful in helping me to ease the pain from the lifelong wounds inflicted upon me in childhood that sadly will never fully heal. My psyche was scarred beyond repair, but I have always embraced diapers -- for protection, for emotional comfort, for sheer fun, for sensual pleasure, and for erotic fulfillment -- and I always will. Diapers help like nothing else.
To echo and to second mamasboy's observation in his really thoughtful and helpful post, lying on my back with my legs up in the diaper position and being changed (hasn't happened for a long time in adulthood, but it has happened, and may happen again with a loving, nurturing yet Dommy Mommy) leaves me in the most vulnerable position imaginable. However, when this happens, I also feel completely at ease, as well as filled to bursting with excitement, in part because my needs -- needs that are as intimate and as personal and as significant as any I might ever have -- are for a time being met. I always reciprocate with endless loving attention and sexual gratification, so there is no equation of being diapered with being narcissistic or selfish or permanently immature. Some planned immaturity with diaper-changing, whether it is regressive or not, helps me to sustain healthy dynamic emotional equilibrium and makes me a stronger and a better man and lover and companion. The contrast between sensitivity and strength yields a complementary dialectic, not mutual exclusivity. It takes a real man to please his woman, and to be courageous and be upright and be strong and, one hopes, honorable and mature and decent in his dealings with others in quotidian existence, and yet to submit or to relent or to yield to his inner child or to his innermost, oft-deemed-forbidden desires, and to allow his woman to diaper him, whether as a nurturing Mommy, a Dommy Mommy, or both.
For me, being an adult-toddler diaperboy and a diaper-lover and incontinent was fused in the crucible of infancy. Again, that has nothing to do with anything illegal or immoral or illegitimate. All that means for me (and, I hope someday, for my Mommy) is that the loving comfort and sensual delight and reassurance and excitement and erotic stimulation that diapers provide is all of a piece, as it always has been. Diapers have always soothed me and diapers have always turned me on. It will always be so for me. I love wetting and soiling and being changed. I love being washed with a facecloth wetted with warm water and coated with Ivory soap. I love being accepted and praised and even loved for being wet and messy, and I love being cleaned and washed and wiped. I love Desitin and baby-powder, and I love being pinned in a clean cloth prefold diaper (birdseye with terry being an especially favored combination) and then having my plastic or rubber pants pulled up. I love many other things too, including kissing, cuddling, holding hands, and making love, and I also enjoy many other activities while I'm in the diaper-position for my girlfriend and Mommy.
I do know why I love diapers so. Having an eidetic memory is a blessing for me in this regard. I know I'm healthier and happier for needing my diapers and for having them and loving them and wearing them and having them changed. But for many, the genesis of the diaper-fetish or being an adult-baby or a diaper-lover is shrouded in the mists of the past. So long as an adult wears baby or adult diapers without illegitimacy (meaning legally, without excessive exhibitionism, without harm to other adults, and with no consideration of minors), it's up to the diapered person to decide whether exploring the psychological and emotional reasons for wearing diapers is worthwhile. Certainly the endless joy they can bring is reason enough to wear them. Some of us wet and mess, some just wet, some do neither. Some of us regress, some don't. Some of us need diapers for medical reasons; some of need diapers for psychological, emotional, or developmental reasons; some of us just love the way they feel and work. The injunction against wetting or soiling and the directive to toilet-train in our society are so overpoweringly strong that many of us feel a frisson of transgressive naughtiness when we buy diapers, speak or write about them, wear them, use them, or have them changed. They can be part of a D/s or Daddy/daughter/son or Mommy/son/daughter dynamic. They can be used for punishment (though I would never want to be punished by being "forced" to wear diapers, or for wetting or soiling; those are hard limits for me, but to each his or her own), or just to augment, heighten, or deepen arousal, humiliation, or submission. Or they can be used to comfort and nurture and reassure those of us who need that so profoundly. Diapers and diapering can do it all.
I really like mamasboy's post. He covers a lot of ground very clearly and succintly and thoroughly. I will write that while I love my ab/dl brother and sisters, and ab/dl Mommies as well (I am a switch, so I extend my courtesies to my fellow Dominant Daddies as well), I have little in common with them except for our humanity and our interest of one sort or another in diapers. There are as many forms and flavors of the diaper-fetish as there are persons who wear diapers. I love and respect them all.
I close by reiterating that I myself took no offense at your question (not that anyone else did either), and that I have welcomed this opportunity to adumbrate a meaningful reply. Feel free to send me a private message if you like. For now, I close by quoting The Glimmer Twins:
"Love is a mystery/I can't demystify." --df