Here's my 2 cents:
I've thought about it a lot and I feel like it's pointless thinking that "oh, if I didn't get exposed to X at this age, I wouldn't have this fetish." I feel that there's a big misconception about this kind of logic that affects people's perception of how fetishes are acquired, which almost makes it out to be far more absurd and arbitrary than it has any right to be. The way that people talk about acquiring fetishes at a young age, they speak about it as if the moment they watched passion patties, a coin somewhere in the universe was tossed, forever deciding whether or not it's going to be what makes them horny for the rest of their lives.
In the interest of finding a less arbitrary model, fetishes - and therefore fetish fuel - should be viewed more for what they are: fantasies, ideas that reach us at a personal, subconscious level, enough to make us want to obsess over them, for them to define the language of our sexuality. For these ideas to reach us so deeply, they have to appeal to us in some way, otherwise, we wouldn't bother obsessing over them and "letting them in" to our minds. What "lets them in" isn't the fetish fuel alone, but our subconscious desires, which are created over time as a result of our individual life experiences. These experiences can differ from person to person, but they're what ultimately decide whether or not we're turned on by certain things more than others, whether we prefer air or liquid, whether we consider ourselves inflatees or inflators, etc.
In other words, forces both within and outside of our control weather our mind, spirit, personality, what-have-you and mold it into any number of "shapes". Certain fetish fuel that we come into contact with happens to be "shaped" in a complimentary manner to the shape of any would-be fetishists' "spirit", and upon contact, they "fuse". In this way, it's not the fetish fuel that makes the fetishist, but the other way around.
The exact reason why any fetish appeals to us is, as stated, deeply personal, but some of the underlying ideas which I've heard people say they find most appealing about inflation specifically are wide-ranging. For some, it's the helplessness and humiliation, it's the punishment angle, it's the sensory aspect of being full or of feeling something soft, of feeling heavy or light (depending on your inflation preference), and for others, it's a combination of many or even all of these. Whether we consciously understand them or not, these underlying ideas are picked up on by our subconscious and are determined worthy of staying, in accordance to how well they suit our subconscious needs/desires. So, if you want to ask yourself, "why on earth am I stuck with this fetish?", a good way to start answering that might be to ask yourself, "When was the first time I remember being turned on by something fetish-y? What do I find appealing about it now, What might I have found appealing about it then? Why might I have found it so appealing? Why did I find it so appealing that I 'let it in' to my mind?"
When it comes to fetishes that aren't possible in real life, it's best to try and think of them as metaphors. I've heard some people speak of connections they've identified between things such as their parent's divorce and their desire to become an inflatee, with the fantasy of inflating appealing to those desires because either the act of inflating made them imagine a more comfortable form of the helplessness that they were feeling, or because the attention that they imagined they'd recieve from being so huge and helpless satisfied the feelings of neglect that they had felt at the time. Obviously, the "why" answer isn't as traumatic or clear-cut for everybody, but this definitely exemplifies a case wherein a traumatic life event created a subconscious need which the mind was able to satisfy through the acquisition of a suitable fantasy, one so enduring that it becomes a full-blown fetish.
Of course, all of this is just a theory, but it's one which I personally prefer, because I feel it gives me more peace of mind to see the process of fetish acquisition as a two-way interaction between the fetishist and fetish material. To believe that the fetish content alone is what "warps" our otherwise "pure spirit" would be to submit to a sort of powerlessness, and to shirk the responsibility of all of our own actions that contributed to the shaping of us up to that point.
I suppose that even with this theory, there is a certain degree of absurdity and arbitrary nature to it - but that cannot be avoided. It is still possible that if you weren't exposed to whatever media gave you a certain fetish, that you wouldn't have ended up with that fetish at that time. Perhaps you would have still ended up with the fetish but at a different time, or perhaps you would have ended up with a different fetish that would have ended up suitably satisfying the needs of your subconscious. Ultimately, you can't go back and change the past, you just have to do whatever is in your power to understand and come to terms with it. This is just what i've had to do in order to achieve that.
Tl;Dr - I find theories of psychology a comfortable and practical model for understanding the "meaning" of my fetishes, and maybe you can too.