When traveling about, it becomes apparent that the display of breasts varies with the climate. After much thought about the subject I have come to the conclusion that this is largely a part of opportunity and desensitization.
In the warmer climates, conditions for favorable breast display may occur at any time of the year. Much of California's year allows for short sleeve shirts. However, this is not the case in the midwest when only a short period of the summer has conditions for breast display. Furthermore, conditions in the central part of the United States during the short summer are often more conducive to breast display than those of the coast regions (higher temperatures). The winters of the midwest are not favorable to breast display unless one is quite fond of Jack Frost nipping at one's nipples.
On the viewer's side, moderate conditions year round breast display lead to many (but not all) women wearing clothing that expresses the breast. As with any stimulus, this becomes discounted after a period of time (and almost expected).
For the those with breasts, almost year round short sleeve weather leads to apathy when displaying the breasts - there is no real reason that breasts should be displayed today, when they can be displayed a month from today too.
On the other hand, in the regions with a short (and intense) summer, many women (and guys if you're into shirtless guys) take this opportunity to flaunt what they have. When there are fewer days to wear the clothing, more women choose to wear the clothing. Walking down a college campus during summer session in the midwest should have warnings for those with failing hearts (be it medical or emotional).
Do realize that there are places where breast display is 'mandatory' from the culture - primarily, New Orleans during Mardi Gras and Los Angeles year round. Traveling abroad to tropical islands also has 'mandatory' breast display.
It is indeed good to travel at times.
The inspiration for this came primarily from experience from the years living in Madison, Wisconsin and a trip that I had taken back to Madison after living in California for 4 years. However, I must admit that while I was a student in college, The Onion published an article titled: "Area Students Prepare Breasts for Increased Springtime Display." The text for this wonderful article can be found at http://www.theonion.com/onion2915/breasts2915.html
While some of the ideas are similar (including the tone) I assure you that at the time of the writing I did not consult The Onion and only regarded the natural specimens as they walked past on the street. At no time did I get slapped.