1) in our culture, we cover up breasts which makes them more interesting & arousing than in other cultures where you might see breasts all the time.
2) A sign of fertility, but NOT in the way most people think. Big breasts do NOT mean more milk production, research has not found any correlation there. It also is likely NOT due to remembering breastfeeding as a baby since you don't form long term memories then, and even if you did, that would make women equally as aroused by boobs as men are.
There are different theories out there... one is that big breasts would be a sign of fertility in that the fattier, plump tissue indicates something that is more "filled out" - hence, younger and healthier (though not necessarily better able to produce milk). Also, one prevailing theory is that men like boobs because boobs look similar to butt cheeks. Most species mate from behind, making the butt such an arousing zone - we're one of the only to mate face to face. Also, a large butt, aka "child bearing hips" DOES mean better ability to bear children without issues.
Interestingly, as much as most men love boobs, I've noticed in any poll I've seen (including one on a p*rn site that was clearly dominated by horny men!) that the majority of men claim to be "ass men" and have a greater preference for curvy hips over a large chest. So while there are plenty of men who prefer boobs, most do seem more turned on by the toosh.
I think it's for a couple reasons. One, they don't have boobs like females, so I guess it's kinda like a curiosity thing. Two, it's instinct, that's how we were brought up, back in the day women were looked at and/or married because of their ability to breed, and it kinda stuck. Three, they are private, covered, it isn't acceptable for a girl to walk around without a shirt, like it is for guys. So I would say, it's instinct, human nature, and curiosity. If you have seen one set of boobs, you haven't seen them all because they are all different.
i already explained that once.. is purely isntict..
bigger boobs means more milk more milk means increase survival rate of babies, more babies means she is a good breeder good breeder makes a good partner to mate and insure your dna is passed to next generation and you playing your part in insuring the surviability of your species..
basically remains of our animal instict that society wanst able to remove..
same goes for why we like girls with big ass/hips ( again theoretically she would be a good breeder)
Btw I'm using the term breeder...to give a better notion that this is a purely animal thing..not that I look at womans as breeders ...