It's an instinct. The "why" doesn't apply to something that's inherently good about boobs, but rather to why thinking of them as good ended up coded into our instincts in the first place. It's a bit like asking why sugar tastes good, or what is it that's so attractive about warmth to a freezing person: the sensation might be the topic of poetry, but the real explanation is in our evolution.
As for that reason, there are a lot of possibilities, but certainly they're good indicators of fertility and a successfully matured female reproductive system, and also of being well fed (if you starve yourself, one of the first places you'll start consuming body fat is from your breasts, since that fat isn't useful if you're not going to raise young).
(An interesting aside is that starving men will more readily break down muscle and brain tissue for nutrition than starving women, who will go after her reproductive fat resources first - an interesting parallel, given that muscle on men is often regarded with the same sexual interest as reproductive fat on women - apparently when we're starving, we figure the sex-getting tissue is expendable.)
(Another random aside is that starvation diets are stupid, for both genders. For women, if you try to starve yourself skinny, you'll starve yourself flat-chested and flat-*ssed long before you go after that stomach - for men, you'll literally starve yourself stupid (and scrawny).)
· 10 years ago