Not sure, I'm not an expert but I think psychologically the development of 'fetishes' is similar to other types of addictions, at least in the psychological aspects; smoking for example is physically addictive but is also psychologically addictive (allegedly smoking resembles comforting memories of suckling a mother's teat or a pacifier as a baby, which is why nicotine replacement alone doesn't cure the craving).
Supposedly fetishes and obsessions might be related to 'magical thinking', or the mind associating some 'quasi-magical' significance to a specific object, activity, body part.
As far as why specific fetishes develop, I'm not sure; might be related to memories of childhood security or something.
I think that breast fetishism, at least taken to an abnormal extreme (e.x. breasts the size of elephant testicles that you see in a fair amount of hentai porn) might be related to unresolved Oedipal tendencies; the obsession with breasts which are abnormal sizes that don't even exist in real life, is likely related to comforting memories of when one used to suckle their mom's teat as an infant.
And my day-to-day observations are that guys who are momma's boys or 'beta males' tend to be fixated specifically on boobs moreso than other guys on average (e.x. as opposed to ass or legs).