The Erotic Highway
You DON'T want to hear me sing like Bob Hope! 
(The louder the groan the better the pun!)
Maybe the left nipple is prefered because it is closer to the heart.
Then maybe smaller breast are more sensitive because they have a higher density of neurons than larger breasts.
Not sure, but I am continuing my research!
TMT Secretly worshipping the sacred feminine in Seattle
How yummy of you BBrain, to be so generous about your nipple sucking! Mmmm....try this one: Start with one nipple. When you get the signal to switch, do it. Repeat procedure with different lady. Keep going until your tongue (or wallet, if paying) falls apart. No, just kidding :-. Seriously, all women are different. I seem to favor my left nipple over my right one, but after a while, I do want to switch. And then go back and forth. Remember though, that some women have had some mammary enhancements and that such procedures can render the entire areolar area numb and insensitive, either temporarily or permanently (not great, but it happens.) So in such a case, one nipple may be more sensitive than the other. And honey, keep the Anal out of this area...lest you'll mistakenly be called an ***hole 
Luvya, Nippleholic,
Love Goddess
You DON'T want to hear me sing like Bob Hope!
(The louder the groan the better the pun!)
Maybe the left nipple is prefered because it is closer to the heart.
Then maybe smaller breast are more sensitive because they have a higher density of neurons than larger breasts.
Not sure, but I am continuing my research!
TMT Secretly worshipping the sacred feminine in Seattle