*cracks knuckles* Aight, lemme see if I can't break down this mystery and make that piece of shit psych degree useful for once.
First, you gotta know the different kinds of inflation. There's the air-based inflation (that our resident Dobson is so fond of), your standard fat-based inflation, and sexual fluid inflation are your main types. Various subtypes will manifest under these main three, such as the blueberry phenomenon or specific bodypart inflation (tits and ass, etc.).
While at first glance it seems that the reasons for the fetish are tied to the subject being an being an overweight outcast and thus trying to love their body (kind of like Fat Acceptance but somehow so much less obnoxious), and no doubt there's probably some of that going on, knowing each of the subtypes means knowing the different reasons that more typically tickles their fetishist's pickle.
(Note that there are often even more fetishes that intersect with inflation, such as hyper, crushing, stuffing, vore, and giantism, but those are all whole different kettles of fish on their own and I'm not going into the psychology of every deviant fuckup or we'll be here all day.)
-Air inflation: This one is almost always tied to cartoon physics and more 'toon'-obsessed furs. This is where you'll most frequently find the legit 'tism; as an obsession with the cartoons of their youth crosses with some burgeoning sexual wires in the brain, and...well. Weird shit happens, often with bicycle pumps. To them, they get their jollies imagining being filled or filling someone/something with air until it floats away, or even more likely, pops. You will also frequently see hose/water-based inflationists among this subgroup, but they're far less common and their goal is not popping so much as filling someone until they are 'leaking' (and you don't wanna know from where).
-Fat inflation: The most common type, this is where you get your standard character ballooning up through eating copious amounts of food phenomenon. These types are typically seeking some kind of comfort from their fetish. Fatness is equated with softness and 'cuddly' aspects; with having 'more to love.' You'll find a lot of projection in these fetishists, with feederism/feedee relationships running rampant (in which they want their partner to be so fat as to be dependent on them OR want to become fat themselves so they can have that 'nurturing' caretaker). This is also, unfortunately, where you'll also find slobfurs who revel in the fatness as a side-effect of living like (sometimes literal) pigs and the sloppy eating that comes with it.
-Sexual fluid inflation: These ones are almost always the sexually inadequate or have fantasies of being more capable than they actually are. They often want nothing more than to be so 'virile' they can produce literal gallons of fluid (most frequently cum but there's a few piss deviants in there as well) that it fills their partner to the point of swelling. Sometimes they will even obsess over the idea of impregnating a partner with this fetish (a sort of 'no kill like overkill' mindset/drown the fucking womb in spawn idea), or just drowning people in the nasty mess. Of all the inflation fetish groups, these creeps are almost always without a doubt virgins and/or not getting laid anytime soon, which is probably a blessing, all things considered.