The fetish is different for everyone, so no two people will answer the same. Often, it grows out of the fear of balloons popping. The human mind closely relates risk to sexual excitement, so a childhood fear can actually turn to fetish at puberty. Sometimes, much later in life as well. It may manifest itself in wanting to be close to balloons but avoiding the pop at all cost (non-popper) or, destruction of the object of the fetish in an attempt to challenge yourself. (popper)
Some people may just be attracted to the beauty of a shiny balloon! Others like the feel of a squishy balloon.
One theory, especially if you are a BTP fan, is that the whole experience parallels the male sexual experience. (Soft, swells up, gets real hard and big, then explodes in climax!)
Whatever the reason, you are not alone! Compared to many fetishes, the balloon fetish is very popular, and does not involve the sexual put-down that plays into some other fetishes.
Enjoy!!