Why are breast taboo?

In North America and in many other westernized countries woman's breasts are a taboo: they are supposed to be hidden. Exposing woman's breasts in public is considered shameful and inappropriate ("indecent exposure") even on beaches or while breastfeeding. In other words, breasts - or at least the nipple - are supposed to be covered at all times.
Parents tend to teach their children the same way, and many times children don't see a single naturally nude breast (apart from their own) while growing up. It is totally possible that a child grows up in North America and never sees a baby breastfeeding!
However, while exposing breasts in public is a 'no-no', children often see breasts (although never the nipples) displayed in a sexually provocative fashion on television, in print media and at magazines. Breasts are emphasized big time in the media and advertisements, who signal to our young people that breasts are a sexual object, and only beautiful when big and protruding.
But we know big breasts is a fallacy since women in ads and movies use push-up bras and breast enlargement to artificially 'enhance' the way their breasts look like. Just think: if young girls continually see this propaganda without a balancing view of natural naked breasts, it is no wonder they also start seriously worrying about the size and shape of their breasts even to the point of suicide!

Breast obsession and men
While women are obsessed about their breast size and shape to the extent of surgery, some men have definite obsessions with female breasts are a source of sexual fantasy / turn-on.
For these men, it is not just a general appreciation of the female figure or the various body parts of a woman, but a fetish, an extreme fixation or obsession. They get sexually inspired by looking at pictures with cleavage, where breasts are enticingly almost visible, yet hidden.
While these men probably realize that this is happening, and feel they enjoy it, they are nevertheless "bound" by this behavior.
The general breast taboo and the provocative media advertisements simply encourage this. Men are being culturally conditioned to see breasts as sexual objects - as almost like inanimate objects that automatically 'click men's brains' to the 'turn on' mode.

Other cultures
European women commonly go topless on beaches, and many European countries also have nudist beaches where people sunbath naked, yet the atmosphere is decent and non-sexual and people are at ease. The sauna culture in some European countries makes people used to seeing nude bodies, and they don't view breasts as any special thing or taboo. Primitive tribes in hot climates wear very little clothing and it's no big deal to them.
Naturists from all over the world always emphasize how sexuality does not equal nudity; nudity is not sexually stimulating if the atmosphere and attitudes are non-sexual. In fact, seeing bare breasts on a topless beach is able to de-sensitize men's minds and take away the "breasts -> sexual stimulation" connection.Clellan Ford and Frank Beach's Patterns of Sexual Behavior came to the conclusion that the most universally admired feminine trait is plumpness -- not perfect breasts. It seems that North American culture (and those influenced by it) are unique in erotizicing the breasts to such an extreme.

Of course breasts are a female body part, and there's nothing wrong in considering them beautiful as part of a woman's body. But let's let breasts be like legs and hips and neck and face etc. and all the other body parts - not some almost like inanimate objects that automatically 'click men's brains' to the 'turn on' mode.